500psi T-Bird Calipers

[chuzie]

Trying to troubleshoot the T-Bird calipers on my 77. They SUCK, but why?

Running T-Bird up front, Explorer discs in back, 1-1/8 MC and WH Hydroboost.

2500 psi @ MC
1500 psi @ transition from hard line to caliper flex with one caliper disconnected. (note this it not at the tee above the diff but at the junction near the coil spring bucket.
500 psi @ caliper bleeder

Tried bleeding with caliper off knuckle and bleeder at 12 o'clock to ensure no air = still 500 psi
Soft line not kinked.

What are my potential culprits here? No much left but flex line, banjo and caliper.

Considered a partial blockage in the line, but that should eventually yield more than 500 psi with repeated pumps, no?

This setup has NEVER worked well in the 10 years I've been running it but I've finally had enough and willing to figure it out.
FWIW- This setup is NOT from a vendor. They are off the shelf parts so I milled the calipers for the banjo and drilled the banjo for the fitting.

EDIT: Summary to date: With a NEW caliper sitting on the inner fender inches from the MC with steel plates sandwiched inside (no pads), connected DIRECTLY to a NEW MC via a NEW hose and a 10" hard line, it yields 500psi, a loss of 2000psi!

 

[Apogee]

FWIW, the bleed screws on those calipers need to actually be pointing almost horizontal in order for the intersecting point between the bleed screw port and the piston bore are at the highest point. Do your calipers look like the ones below? When you say that you located the bleeder in the 12 o'clock position, do you mean like in the image below or rotated CCW about 90 degrees?

Does your '77 have the flex lines between the MC and combination valve? Does it have the factory combination valve? Age doesn't typically do rubber hoses any favors with respect to system compliance, but you've noted that they've sucked since day one, so that's probably not it. That said, a partial blockage in either the valve or a hose (front center hose maybe) could be throttling the fluid and therefore knocking down the pressure being supplied to the calipers. When bleeding the front calipers, do you get a nice stream of fluid when you crack the bleeders or is it more of a trickle?

What does the pedal feel like when applying the brakes? Is the pedal high and hard, or soft and mushy/spongy? The WH hydroboost should be a kit from Hydratech Braking, which uses genuine Bosch units from the mid-aughts Silverado pickups IIRC, so the likelihood of it being the HB unit is low, but not impossible...that doesn't explain the pressure drop between the MC and the caliper though, as that should be minimal. With pressure testing I've done on my cars, I don't usually see more than a 25-50 PSI drop between the MC and caliper, so something is up.

That said, with the T-bird calipers (Ø3.110 bore & 7.59 sq inches piston area versus the factory Ø2.88 calipers with 6.51 sq inches, approx 15% less area) at 500 PSI should create 3796 lb of caliper clamping force, which is not insignificant. For most applications, that's front tire lockup territory on the street, but it ultimately depends on your setup. The Explorer rears calipers have Ø47.9mm [Ø1.88"] piston diameters, which equates to a 2.77 sq inch effective piston area, and typically do well with the smaller OE front calipers on EB's, though it depends on the EB due to their short wheelbases and higher CoG's, especially if lifted.

Without knowing what the pedal feel is, I can't be super specific as to what I'd do, but I'd be looking at getting braided stainless hoses, as it would then rule out any potential rubber hose issues and should improve system performance regardless, so it's a win-win whether the rubber hoses are the issue or not. If the pedal is high and hard, my next thing to check would be the brake pads, as they can make a huge difference with respect to brake feel and overall brake torque. They don't need to be expensive pads, but they need to work as intended...I tend to avoid anything that provides a "lifetime guarantee" from an auto parts store and have some preferred brands, but a decent daily-drive grade semi-metallic or hybrid-type ceramic usually does the trick in my experience. If the pedal is soft, spongy and/or "pumps up" with repeated applications of the brake pedal, then it sounds like either a bleeding issue or an MC issue. Are you running the Ø1-1/8" bore C3 Corvette-based master cylinder supplied with the hydroboost? Is it plumbed front to front, and rear to rear? If the pedal is squishy, I'd probably disconnect both brake circuits from the MC and plug both outlet ports. I would then ensure that the MC is fully bleed and doesn't leak down over time with steady pedal pressure. Once that's done (essentially a plugged-port bench bleed on the car), I would connect only the rear brake circuit and then bleed it out (use the clip on the combination valve if running it to keep it from shifting) until I had a good pedal. Connect the front brake circuit and repeat.

 

[jamesroney]

Trying to troubleshoot the T-Bird calipers on my 77. They SUCK, but why?

Running T-Bird up front, Explorer discs in back, 1-1/8 MC and WH Hydroboost.

2500 psi @ MC
1500 psi @ transition from hard line to caliper flex with one caliper disconnected. (note this it not at the tee above the diff but at the junction near the coil spring bucket.
500 psi @ caliper bleeder

Tried bleeding with caliper off knuckle and bleeder at 12 o'clock to ensure no air = still 500 psi
Soft line not kinked.

What are my potential culprits here? No much left but flex line, banjo and caliper.

Considered a partial blockage in the line, but that should eventually yield more than 500 psi with repeated pumps, no?

This setup has NEVER worked well in the 10 years I've been running it but I've finally had enough and willing to figure it out.
FWIW- This setup is NOT from a vendor. They are off the shelf parts so I milled the calipers for the banjo and drilled the banjo for the fitting.

Is there a metering / proportioning / imbalance valve somewhere in the circuit?

How do YOU explain that you have 2500 psi at the MC, and 500 psi at the bleeder? Where did the other 2000 psi go?

Even if you have air in the system...if you have 2500 psi at one end of a pipe...you should be able to find at least 2450 psi at the other end of the pipe.

You have a restriction somewhere. My guess is that you have the front brake circuit plumbed into the rear, or a shuttle valve blocking your flow. When you bleed the brakes, do you get the full displacement of fluid coming out the bleeder? (figure 1-1/8 bore x 1 inch stroke ~ .99 cubic inches of fluid per stroke)

and no...if you have a blockage in a line, repeated pumps will not increase the pressure downstream of the blockage.

Post a pic of your set-up.

 

[chuzie]

do you mean like in the image below or rotated CCW about 90 degrees?

Just like in the picture. Rested on top rotor with wood in caliper.

Does your '77 have the flex lines between the MC and combination valve?

No, hard lines from MC to hydraulic line lock then down to the junction on the frame and braided line to the junction atop diff.

Does it have the factory combination valve?

Nope

When bleeding the front calipers, do you get a nice stream of fluid when you crack the bleeders or is it more of a trickle?

Definitely unimpressive volume. Takes a good 10 cycles + to fill up one of those tiny bottles you get with a bleed kit. Maybe 6 oz?

What does the pedal feel like when applying the brakes?

Soft / mushy till it gets to bottom of travel then slowly creeps back to return position.

The Explorer rears calipers have Ø47.9mm [Ø1.88"] piston diameters, which equates to a 2.77 sq inch effective piston area, and typically do well with the smaller OE front calipers on EB's, though it depends on the EB due to their short wheelbases and higher CoG's, especially if lifted.

They have an adjustable prop valve tuned to prevent rear lockup

Without knowing what the pedal feel is, I can't be super specific as to what I'd do, but I'd be looking at getting braided stainless hoses.

Everything is braided stainless but couldn't hurt to swap em out.

They don't need to be expensive pads, but they need to work as intended...

Running Rayestos Element3 PGD50M https://www.summitracing.com/parts/AGB-PG-D50M

Are you running the Ø1-1/8" bore C3 Corvette-based master cylinder supplied with the hydroboost? I

I believe so... CPP M-C118-L https://www.summitracing.com/parts/CLP-MC118-L

is it plumbed front to front, and rear to rear?

Rear port to front brakes

If the pedal is squishy

Sure ain't hard

I'd probably disconnect both brake circuits from the MC and plug both outlet ports. I would then ensure that the MC is fully bleed and doesn't leak down over time with steady pedal pressure. Once that's done (essentially a plugged-port bench bleed on the car), I would connect only the rear brake circuit and then bleed it out (use the clip on the combination valve if running it to keep it from shifting) until I had a good pedal. Connect the front brake circuit and repeat.

Will do. Thanks for your comprehensive reply. I always appreciated your expertise on the subject.

 

[chuzie]

Is there a metering / proportioning / imbalance valve somewhere in the circuit?

Nothing for the fronts other than a hydraulic line lock but the 1500 psi was downstream of that device.

When you bleed the brakes, do you get the full displacement of fluid coming out the bleeder? (figure 1-1/8 bore x 1 inch stroke ~ .99 cubic inches of fluid per stroke)

I don't think I'm getting that much.

Post a pic of your set-up.

First thing tomorrow. Thank you very much

 

[jamesroney]

Nothing for the fronts other than a hydraulic line lock but the 1500 psi was downstream of that device.

Defeat the line lock with a jumper tube. Then you should see the full line pressure at the caliper.

I don't think I'm getting that much.

I don't think so either. But easy to verify with a 3/16 clear hose and a cup. Bleed your brakes at the line lock location into the cup. Measure what 3-4 good pumps delivers. Compare that to what you see coming out the bleeder at the caliper.

First thing tomorrow. Thank you very much

Best of luck. With your set-up...you should be launching yourself into the windshield with minimal pedal effort.

 

[Broncobowsher]

When you were getting the 2500 at the master, how did you measure it?
Was a T-fitting installed and that is where you measured it at, with the rest of the system still in action?
Or did you completely remove the line and screw only the pressure gauge into the master, so the only thing in the circuit was the gauge?
If power brakes, did you recharge the booster after every test (run the engine)?

 

[ba123]

Just an fyi, if you ever replace those calipers and don't want to remachine for the fittings you used, which seems like a royal pain and could even be your issue, who knows...

I have TBird calipers up front and these hoses:
https://www.amazon.com/gp/product/B01MTY8XX5/?tag=classicbroncos-20

If those aren't right for you, I'm sure you could find the right ones instead of machining a caliper. You might have to change the fitting (and reflare) the hard line but that's not the biggest deal.

Also, HOW you bleed is important, imo. Sometimes air gets stuck in lines and you just can't get it out unless you pressure bleed. I also have the WH Hydro, I bled it one time and it is fantastic.

I would bet you have a kink somewhere or you have a fitting that isn't perfect and letting air in and a little fluid out but not enough to be obvious. Touch under every single fitting every time you test the pedal.

 

[chuzie]

Or did you completely remove the line and screw only the pressure gauge into the master, so the only thing in the circuit was the gauge?

This ^^^

If power brakes, did you recharge the booster after every test (run the engine)?

Yes

 

[chuzie]

I'd probably disconnect both brake circuits from the MC and plug both outlet ports. I would then ensure that the MC is fully bleed and doesn't leak down over time with steady pedal pressure. Once that's done (essentially a plugged-port bench bleed on the car), I would connect only the rear brake circuit and then bleed it out (use the clip on the combination valve if running it to keep it from shifting) until I had a good pedal. Connect the front brake circuit and repeat.

Did a leak down on the MC with and without assist applied with no pressure loss after 30 seconds.
Bleeding accomplished as instructed, no change

Defeat the line lock with a jumper tube. Then you should see the full line pressure at the caliper.

Same - 500psi @ caliper

When you bleed the brakes, do you get the full displacement of fluid coming out the bleeder? (figure 1-1/8 bore x 1 inch stroke ~ .99 cubic inches of fluid per stroke)

But easy to verify with a 3/16 clear hose and a cup. Bleed your brakes at the line lock location into the cup. Measure what 3-4 good pumps delivers. Compare that to what you see coming out the bleeder at the caliper.

Here are the average numbers from 5 pumps to each corner and the line lock in cubic inches. Keep in mind this isn't going to be exact due to minor variation in pedal travel each time but should be close...

RR : 0.85
RL : 0.79
FR : 0.73
FL : 0.54
Line Lock : 0.61
Note: The FL caliper is not installed on the knuckle but does have a block to hold the piston.

I think the reading from the line lock was inaccurate since I was having to deal with a loose line and use my finger to plug it upon pedal release. Can't fathom why it would yield a number lower than the calipers it supplies.

 

[Broncobowsher]

This ^^^

Yes

What you did was bypass the entire plumbing system when you placed the gauge directly into the master. You eliminated all the expansion in all the lines. At this point you proofed the master can make the pressure.

As for the pressure drop, it is coming from flex. I've heard of hard lines flexing (expanding) under pressure. This was typically when someone tried to run copper instead of steel.
The rubber lines could be flexing, stretch like a balloon.
Air inside the caliper could be compressing.
The pad material could be compressing (bad pads).
I doubt the caliper is flexing, but if cracked, super long shot.

What it isn't, a seal. You are not leaking fluid. It can't get out then suck itself back in. Physics prevent that. Not even an internal seal in the master, you proofed that out.

The 1500 with one caliper deleted above the flex line, that is telling. It is sounding like flex line or downstream is the issue. You loose half your expansion, you keep more of the pressure. Not all, since the higher pressure will make more flex.

There is no pinpoint problem at this point. But you are working on the correct diagnostics. Further refinement of isolating parts of the brake system is needed.

 

[chuzie]

The 1500 with one caliper deleted above the flex line, that is telling. It is sounding like flex line or downstream is the issue. You loose half your expansion, you keep more of the pressure. Not all, since the higher pressure will make more flex.

Would be nice if I had a way to put my gauge on the banjo end of the flex line. That would make quick work to identify ballooning of the line or point directly to the caliper.

 

[DirtDonk]

How do you have your master cylinder hard lines routed? I’m curious where that first thousand psi is disappearing to.
Since you’re measuring that before the flex hose to the axle, it sounds like(?), what is it in those first couple of feet of hardline that is costing you 1000 psi.
Granted, you’re losing another thousand just down the line a tiny bit, but it seems like that first loss is one of the key ingredients here.

 

[chuzie]

How do you have your master cylinder hard lines routed? I’m curious where that first thousand psi is disappearing to.
Since you’re measuring that before the flex hose to the axle, it sounds like(?), what is it in those first couple of feet of hardline that is costing you 1000 psi.

I was measuring AFTER the flex to the axle but BEFORE the flex to the caliper (tee above differential).
I initially wondered where the PSI went too, but then realized it was likely being absorbed by the other caliper/flex line since it wasn't capped off during the measurement. Willing to bet if remove both lines from the tee above the diff, cap one side and measure the other, it'll be 2500 psi. Now, that is a dangerous assumption so guess I need to put my money where my mouth is and get back to the shop!

 

[ntsqd]

Sliding calipers flex under pressure. I have watched that occur in a lab setting where we were measuring volumetric displacement vs pressure (i.e. how far do you need to move the m/c piston to achieve a set psi with each model of caliper). They are the worst (best) caliper design for this "feature".

Even steel lines grow under pressure. Not much, but sometimes you can feel it and occasionally you can see it.

Curious if you've done anything like disconnect at the calipers and at the m/c, and then tried to blow compressed air thru in either or both directions.

 

[chuzie]

Curious if you've done anything like disconnect at the calipers and at the m/c, and then tried to blow compressed air thru in either or both directions.

I haven't, but the thought crossed my mind. I may disconnect both calipers and their associated soft lines first. If I get full MC pressure, I'm just gonna toss the lines and calipers and start fresh. If not, I'll move up system to the frame-axle soft line and do the same. If that doesn't yield a positive result, gonna hook up shop air at 20psi and break out the anemometer!

 

[DirtDonk]

Are there any adapter fittings in the system? If so, I wonder if any of them are defective and have incorrect, or faulty passages in them.

 

[chuzie]

Are there any adapter fittings in the system? If so, I wonder if any of them are defective and have incorrect, or faulty passages in them.

Only adapters are on the MC ports.

---

2500psi

That's what I just measured at the tee above the differential with both hard lines going to the calipers removed, one side plugged and other side gauged.

This eliminates everything but:
- Hard lines from tee to caliper flex lines
- Caliper flex lines
- Calipers
- Pads

I think all I can do at this point is gauge each hard line extending from the tee individually, then try testing airflow through the lines. After that, time to start changing parts.

I am NOT a proponent of changing parts until I am sure of the culprit. Tossing stuff at the wall to see what sticks drives me nuts, but I'm not sure I have much choice in this matter without investing more money in diagnostic equipment than the cost of new calipers and flex lines.

My money is on the caliper flex lines. They are the only constant left in this equation as the calipers have been changed once previously with no change in result.

 

[ntsqd]

What are those flex hoses? OEM Rubber? Braided SS? If braided, bought or built?

 

[chuzie]

What are those flex hoses? OEM Rubber? Braided SS? If braided, bought or built?

Braided stainless set. Bought from a Bronco vendor years and years ago.