500psi T-Bird Calipers

[ntsqd]

I think it's time to flow test them. I'd just use compressed air to see it you can get full flow from one caliper to the other.

 

[DirtDonk]

How many years ago?
Didn’t someone have a bad batch of hoses at one point? It’s a very vague memory, and I might be making it up.
I don’t think it was us, but then again I don’t really remember any details.

 

[chuzie]

How many years ago?
Didn’t someone have a bad batch of hoses at one point? It’s a very vague memory, and I might be making it up.
I don’t think it was us, but then again I don’t really remember any details.

Oh man couldn't even begin to guess who or when. I'll tell you this problem has been present since then, so it's possible! Flow test in a couple days

 

[ksagis]

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

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.

Nope

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?

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

They have an adjustable prop valve tuned to prevent rear lockup

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

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

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

Rear port to front brakes

Sure ain't hard

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

Terminology for MC port location and front/back always a big weird to me. The Vette power brake MC that many of us have used is factory plumbed in the 4 wheel disk OEM config with the furthest forward (closest to radiator) pot going to the front brakes.

Wasn’t clear to me that you and @Apogee were saying the same thing.

And per what @jamesroney said on pressure, for a closed system, it’s not clear to me how pressure isn’t very nearly the same throughout the system (if you hold the pedal down for any reasonable length of time). Even if there is a restriction, seems like eventually it will equalize.

 

[nvrstuk]

Just saw this post for the first time tonight... watching and waiting!

Hope you get it figured out soon Chuzie.

Mushy pedal feel is related to both air in the lines or volume changing (expanding lines) which both affect the pressure.

 

[DirtDonk]

You feel like the rear brakes are doing their share of the work though? I’m just wondering if the master cylinder isn’t in fact smaller than you think. Then again, it would have to be pretty darn small to make it feel like it’s going to the floor. So more than likely there’s a discrepancy somewhere else.

 

[ntsqd]

Ø1.125" M/C with those calipers is going to feel a little mushy even when perfectly bled. A Ø1.25 M/C would be closer to normal feeling, especially with a H-B unit. Pedal feel is very subjective though, so whatever feels good to you is what it should be.

 

[chuzie]

The Vette power brake MC that many of us have used is factory plumbed in the 4 wheel disk OEM config with the furthest forward (closest to radiator) pot going to the front brakes.

Actually, I thought I researched that it was rear port to front brakes. Certainly possible, but do you have anything I can reference to confirm? I can call CPP as well. Will add it to list for Monday.

You feel like the rear brakes are doing their share of the work though? I’m just wondering if the master cylinder isn’t in fact smaller than you think.

Absolutely! Adjustable prop is in full effect to keep them from locking up. I manually measured MC bore = 1.125"

Ø1.125" M/C with those calipers is going to feel a little mushy even when perfectly bled. A Ø1.25 M/C would be closer to normal feeling, especially with a H-B unit. Pedal feel is very subjective though, so whatever feels good to you is what it should be.

I think I prefer a tad mushy on the rocks. Good insight. Thank you

 

[ntsqd]

Easy to get wrapped around the axle over front port vs. rear port. There are some subtle reasons for doing it one way vs. the other. Until you have good, functioning brakes it doesn't matter. After you have those, then can discuss how it performs vs. how you want it to perform. And changing whatever it is now to the opposite is going to be a subtle difference. Might be worth doing, if only just to know how that feels, and it might not be worth the trouble.

 

[ksagis]

Actually, I thought I researched that it was rear port to front brakes. Certainly possible, but do you have anything I can reference to confirm? I can call CPP as well. Will add it to list for Monday.

Absolutely! Adjustable prop is in full effect to keep them from locking up. I manually measured MC bore = 1.125"

I think I prefer a tad mushy on the rocks. Good insight. Thank you

Prepare to be underwhelmed with the rigor of this reply, happy to share what I dug up a couple of years when trying to confirm PO as-installed configuration:

From Hydratech site here, I found these quotes for MC8111 PN: "1/2-20 Front Port - 9/16-18 Rear Port" and "Front Port = Front Brakes / Rear Port = Rear Brakes". I'm pretty sure the 9/16" port called the "rear port" is the one closest to the firewall (I can check the Vette MC in the junk pile tomorrow to confirm)

From RockAuto here:

Primary Port Thread A	9/16"-18
Secondary Port Thread A	1/2"-20

Source defining primary and second ports with primary post being one closest to firewall

And if you put any credence in Youtube mechanic videos, this guy is doing a gravity bleed and when gravity bleeding on rear brakes, the pot closest to the firewall is the one emptying:

YMMV, let us know what CPP says.

 

[DirtDonk]

Many GM vehicles had front to front and rear to rear orientation with this type of master.
I believe even a Ford, or two use this orientation, but by far and away most Fords use rear port to front and front port rear.
In the aftermarket, I’ve heard many argue for this orientation. But I’ve never heard, that I can remember anyway, a single detail as to why it must be that way.
Of course, when you have one reservoir that’s larger than the other, that one almost always goes to a disc brake end, given a disc/drum setup.
Ford started using master cylinders with one larger reservoir, even back when they were drum/drum vehicles. I believe this was in a way of beginning to standardize things, knowing that some vehicles using this master design would get disc front brakes. Even while others using this master still had drums at all four corners.

Early tandem masters on Broncos (‘68 and’69 maybe?) were equal size reservoirs. Kind of in a miniature version of the GM one that we are used to seeing with these setups.
By ‘70 maybe, and definitely by ‘71, we got the larger reservoir in the rear, for the front brakes.

 

[ntsqd]

In a tandem m/c there are two pistons inline with each other. They are not really connected to each other, but there is not much slop between them either. The rear piston has to move very slightly to create a slug of brake fluid under pressure. This slug of fluid pushes on the rear end of the front piston and is what moves the front piston. (Recall that fluids are treated as being incompressible.) Until there is a leak in the rear circuit there is no mechanical connection between the two pistons.

So whichever brakes are connected to the rear port have a very small head-start over whatever brakes are connected to the front port. As I said, maybe this matters and maybe it doesn't. Some vehicles may be sensitive to this and others may not.

When there is a difference in volume of the respective reservoirs it is obvious which the OEM intended to be connected to which, but when you're using a GM M/C on a Ford then those preferences are going to disagree.

Front to front, rear to rear would earn the system a very slight delay in the front brakes being applied. Early GM front disc brake systems included a "delay valve" in the front brake plumbing that made sure that the rear brakes got a head-start on stopping the vehicle. I'll guess by about 1970 those went away. I've only recently become aware of Ford ever having such a valve in their front disc systems.

 

[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.

I stand corrected, there are 3/8 - 7/16 adapters on the caliper hoses.

I think it's time to flow test them. I'd just use compressed air to see it you can get full flow from one caliper to the other.

Snuck out there early this morning. Air flows unrestricted from one end to the other, eliminating possibility of obstruction in the lines.

Suspects remaining,
- Hose expansion
- Bad calipers
- Bad pads

As stated previously, calipers and pads have been changed in the past, but not hoses since the problem arose.
That said, screw it, I'm ordering caliper and hose kit from Duffs!

 

[ntsqd]

FWIW I buy the parts and make my own braided SS brake hoses. I always buy one extra of each unique hose end, and enough extra hose to make the longest hose on the vehicle. These extras go in a sealed bag as trail spares. I've never needed them, but knowing Mr. Murphy I continue to do this and advise anyone else to do the same.

Used to be all of the pre-made hoses had a tiny tube at the junction of the end of the hose to the fitting that was serious weak point in the hose. Normal use would never tax it, but over-extending the suspension could easily break it. Some mfg's have changed their design to address this, but not all of them. The screw-together hose ends don't have this design problem.

I used to apply heat-shrink to the hose to keep it clean of grit as the grit would wear out the SS strands eventually. Now I can buy the hose with a polymer covering already on it. Be sure to buy the hose and the fittings from the same mfg. as particularly with the cover hose there can be a mis-match that makes it not go together correctly.

 

[ksagis]

I stand corrected, there are 3/8 - 7/16 adapters on the caliper hoses.

Snuck out there early this morning. Air flows unrestricted from one end to the other, eliminating possibility of obstruction in the lines.

Suspects remaining,
- Hose expansion
- Bad calipers
- Bad pads

As stated previously, calipers and pads have been changed in the past, but not hoses since the problem arose.
That said, screw it, I'm ordering caliper and hose kit from Duffs!

One more possibility that I don’t think we talked in this thread, if the backing plate and rotor aren’t fully parallel, some portion of pedal travel will be used up getting things into alignment.

If the lines flowed clear, that eliminates obstructions. And, if that’s the case, I don’t see how the pressure isn’t the same throughout system (for a system that isn’t leaking).

The way I think about it, even of there is a line downstream of MC that is expanding, the 1000 psi is going to do work, expand the line, and the pressure is going to equalize throughout the system. If the pressure doesnt equalize, then fluid is going to move from high pressure area to low pressure. An expanding line is just like an accumulator or expansion tank.

What am I missing?

 

[ksagis]

In a tandem m/c there are two pistons inline with each other. They are not really connected to each other, but there is not much slop between them either. The rear piston has to move very slightly to create a slug of brake fluid under pressure. This slug of fluid pushes on the rear end of the front piston and is what moves the front piston. (Recall that fluids are treated as being incompressible.) Until there is a leak in the rear circuit there is no mechanical connection between the two pistons.

So whichever brakes are connected to the rear port have a very small head-start over whatever brakes are connected to the front port. As I said, maybe this matters and maybe it doesn't. Some vehicles may be sensitive to this and others may not.

When there is a difference in volume of the respective reservoirs it is obvious which the OEM intended to be connected to which, but when you're using a GM M/C on a Ford then those preferences are going to disagree.

Front to front, rear to rear would earn the system a very slight delay in the front brakes being applied. Early GM front disc brake systems included a "delay valve" in the front brake plumbing that made sure that the rear brakes got a head-start on stopping the vehicle. I'll guess by about 1970 those went away. I've only recently become aware of Ford ever having such a valve in their front disc systems.

I haven’t looked at seals on MC in a bit, but seems like if the front piston (one closest to radiator) is connected to front brakes, and moving the front piston relies on the rear piston moving fluid, if the oring or seal that creates that movement failed, one would lose front brakes (instead of losing rear brakes for a system with a reversed line config)

Agree on the subtle delay if front pot goes to front brakes.

If my logic above is right, maybe thats the motivation various designers had in mind.

 

[ntsqd]

If the inter-piston seal fails then after it takes up the slack it behaves like it's a single piston, old school m/c. If the rear port's system fails then the rear piston travels a short distance (rebuilding tandem m/c's we used to have to set that distance) before the rear piston crashes into the rear of the front piston and mechanically drives it forward.

A mis-aligned caliper bracket will cause the pedal to have a longer than expected travel due to the caliper piston needing to travel further, but once the hydraulics have 'aligned' the caliper the pedal will feel normal for whatever that system's combo is and the line pressure will be what it is supposed to be. The brakes will also not feel very linear over that longer stroke.

A bulging hose won't behave like a typical hyd system accumulator, it will behave more like a balloon. If the strength of that balloon is 500 psi, then more pedal stroke will just expand the balloon further, it won't increase the pressure.

 

[ksagis]

If the inter-piston seal fails then after it takes up the slack it behaves like it's a single piston, old school m/c. If the rear port's system fails then the rear piston travels a short distance (rebuilding tandem m/c's we used to have to set that distance) before the rear piston crashes into the rear of the front piston and mechanically drives it forward.

A mis-aligned caliper bracket will cause the pedal to have a longer than expected travel due to the caliper piston needing to travel further, but once the hydraulics have 'aligned' the caliper the pedal will feel normal for whatever that system's combo is and the line pressure will be what it is supposed to be. The brakes will also not feel very linear over that longer stroke.

A bulging hose won't behave like a typical hyd system accumulator, it will behave more like a balloon. If the strength of that balloon is 500 psi, then more pedal stroke will just expand the balloon further, it won't increase the pressure.

Copy all, makes sense on the hose. The balloon analogy computed for me.

 

[nvrstuk]

I"m still thinking it's lines. Mushy pedal, never "worked" great, psi drop is very significant because lines/hoses expand thereby increasing volume so pressure drops. Never seen similar symptoms caused by a caliper. Back to my volume and pressure correlation.

 

[ntsqd]

I've lost track of how easily the calipers bleed. If not easy, then I can see maybe there's a problem internally where something is acting to reduce the pressure at the bleed screw(s). Would be a supercalafragilistically weird thing to happen, but weirder has happened. Only thing that could be weirder would be if the pads have become compressible somehow.

But, yeah. The flex hoses are usually the culprit for weirdness in a brake system.