500psi T-Bird Calipers

[ntsqd]

Something that I suspect that is frequently not understood about hydraulics is that everything is scalable. If you increase the caliper piston area by 15%, and then increase the m/c piston area by 15% you've swapped around a bunch of parts for no gain. Even among folks who work with hydraulics somehow think that brakes behave differently. They do not.

What matters is the hydraulic leverage ratio. M/C piston area divided into the caliper piston area. To increase that leverage the m/c bore needs to become smaller (seems to be rarely done) or the caliper pistons need to become bigger (much more common).

Changing from a Ø1-1/8" bore m/c with the OE Ford Ø2-7/8" calipers to a Ø1-1/32" bore m/c will give almost exactly the same results as keeping the Ø1-1/8" bore m/c and changing to the T-Bird calipers. (Where you'd find a Ø1-1/32" bore m/c I have no idea, but that's not the point.)

Excessive pad knock-back is the cause of a low pedal in an otherwise 'normal' set-up. Figure about .015" take-up per brake pad, so roughly .06" times the piston area is the take-up volume required. I just looked at m/c piston stroke vs. this take-up volume for the three common calipers (GM, Ford, & T-Bird) vs. Ø1.00 m/c, Ø1.125 m/c, & Ø1.250 m/c. Only the Ø1.00 used with the two largest piston calipers uses more than 0.5" of m/c piston stroke. If you're running out of m/c piston stroke before the brakes become effective it is not the fault of the m/c bore size. Something else is wrong.

Everything that I've read about H-B units cautions against pushing on the brake pedal while the engine is running without a m/c in place. You will, apparently, forcibly disassemble the H-B unit.

 

[chuzie]

If you're running out of m/c piston stroke before the brakes become effective it is not the fault of the m/c bore size. Something else is wrong.

What about the scenario with a HB designed for a 1" stroke MC that has a 1-1/8 stroke MC attached?

Everything that I've read about H-B units cautions against pushing on the brake pedal while the engine is running without a m/c in place. You will, apparently, forcibly disassemble the H-B unit.

It is this very reason I can't measure the effective stroke of the HB.

Such a frustrating situation. I do understand your point though about swapping parts and essentially achieving nothing.

 

[ntsqd]

I've no idea what stroke(s) are possible in an H-B unit, but since the total stroke of a m/c is a carefully designed feature I doubt that they will have less stroke than any of the OEM m/c's that they're intended to be used with.

I've never tried to push my H-B to it's limit w/o the engine running and no m/c attached. I'm looking at two more H-B conversions in the near future, I'll have to gently try that.

 

[Apogee]

ntsqd is correct about risking damage to the HB unit if you stroke it (under power) without a master cylinder attached, and not just any master cylinder, but one with the proper pushrod hole depth. The output rod length determines the appropriate pushrod depth on the MC, as you want a small amount of free play between the output rod and MC when assembled, .015" to .020" is typical. Paul at Hydratech (former owner) was talking about just that issue to me once where people run the deep pushrod MC with the short output rods and blow out the guts on the units, so it's a real problem. That said, stroking it manual should be fine so long as the accumulator reservoir has been bled off first.

 

[ksagis]

For any of us using a HB directly connected to the OEM brake pedal (like at least me), we're not getting a bunch of stroke out of the HB and MC.

OEM 6:1 pedal with 3 inches of travel is only gonna move the MC 0.5", and that's for a system with little gap in pushrod, etc. It would take 6 inches of pedal travel (for an OEM pedal) to get 1 inch of MC movement.

I'd love to know what pedal ratio is for a factory HB!

 

[ksagis]

ntsqd is correct about risking damage to the HB unit if you stroke it (under power) without a master cylinder attached, and not just any master cylinder, but one with the proper pushrod hole depth. The output rod length determines the appropriate pushrod depth on the MC, as you want a small amount of free play between the output rod and MC when assembled, .015" to .020" is typical. Paul at Hydratech (former owner) was talking about just that issue to me once where people run the deep pushrod MC with the short output rods and blow out the guts on the units, so it's a real problem. That said, stroking it manual should be fine so long as the accumulator reservoir has been bled off first.

Was wondering if you have any thoughts on how detrimental some side load is on input to HB.

 

[ntsqd]

Unless you're running a Ø1.00 m/c with any of the 3 common calipers you won't need all .50" of m/c piston travel to build pressure in the system, and the Ford caliper will likely work with the Ø1.00 m/c but it doesn't have much margin. I'd be really hesitant to run that combo without modifying things so that the pedal can have more stroke.

I look at the push-rod side loading as F*Tanθ where θ is going to be smallish. Maybe 20° max? F, otoh, could be 1200 lbf fairly easily. So roughly 440lbf as a calc'd max from some guesses. If the power piston in the H-B is riding on the PS oil pressure then even though 440 lbf is significant I don't see it causing much trouble.

 

[ksagis]

Unless you're running a Ø1.00 m/c with any of the 3 common calipers you won't need all .50" of m/c piston travel to build pressure in the system, and the Ford caliper will likely work with the Ø1.00 m/c but it doesn't have much margin. I'd be really hesitant to run that combo without modifying things so that the pedal can have more stroke.

I look at the push-rod side loading as F*Tanθ where θ is going to be smallish. Maybe 20° max? F, otoh, could be 1200 lbf fairly easily. So roughly 440lbf as a calc'd max from some guesses. If the power piston in the H-B is riding on the PS oil pressure then even though 440 lbf is significant I don't see it causing much trouble.

Thanks for the reply, I was doing my math based on 150 lbs of foot pressure (maybe high given HB assists in play), pedal ratio of 6:1, and 15 degrees of off axis push. That gives 250 lbs of side force on the input rod assembly.

Looking at attached picture, I’m struggling to understand how side load on input shaft translates to seal wear, but trying to reconcile the various comments that have been made.

Thanks to @catfan, I have a unit to debuild and get more info.

 

[chuzie]

... I doubt that they will have less stroke than any of the OEM m/c's that they're intended to be used with.

That's just the rub here, not only do I have zero information on the origin /application of this HB or MC. The MC, though "corvette style" was from CCP and they have now proven their lack of knowledge on the subject.

That said, stroking it manual should be fine so long as the accumulator reservoir has been bled off first.

That's great news. I'll absolutely attempt to measure stroke.

Looking at attached picture, I’m struggling to understand how side load on input shaft translates to seal wear, but trying to reconcile the various comments that have been made.

I'm not familiar with the side load discussion. Where is that taking place?

 

[ZOSO]

Something that I suspect that is frequently not understood about hydraulics is that everything is scalable. If you increase the caliper piston area by 15%, and then increase the m/c piston area by 15% you've swapped around a bunch of parts for no gain. Even among folks who work with hydraulics somehow think that brakes behave differently. They do not.

What matters is the hydraulic leverage ratio. M/C piston area divided into the caliper piston area. To increase that leverage the m/c bore needs to become smaller (seems to be rarely done) or the caliper pistons need to become bigger (much more common).

Changing from a Ø1-1/8" bore m/c with the OE Ford Ø2-7/8" calipers to a Ø1-1/32" bore m/c will give almost exactly the same results as keeping the Ø1-1/8" bore m/c and changing to the T-Bird calipers. (Where you'd find a Ø1-1/32" bore m/c I have no idea, but that's not the point.)

Excessive pad knock-back is the cause of a low pedal in an otherwise 'normal' set-up. Figure about .015" take-up per brake pad, so roughly .06" times the piston area is the take-up volume required. I just looked at m/c piston stroke vs. this take-up volume for the three common calipers (GM, Ford, & T-Bird) vs. Ø1.00 m/c, Ø1.125 m/c, & Ø1.250 m/c. Only the Ø1.00 used with the two largest piston calipers uses more than 0.5" of m/c piston stroke. If you're running out of m/c piston stroke before the brakes become effective it is not the fault of the m/c bore size. Something else is wrong.

Everything that I've read about H-B units cautions against pushing on the brake pedal while the engine is running without a m/c in place. You will, apparently, forcibly disassemble the H-B unit.

The way I am seeing this is back to the pressure issues. If he is getting only 500psi at the caliper that to me says not enough volume to fully fill the caliper to reach full pressure. If the hydroboost is only stroking the master cylinder say .5" when the mc is a 1.2" stroke you are only moving so much fluid. This was evident in the test where he locked the line lock and pumped the brakes multiple times and got 2K psi at the caliper. Line lock is not letting the fluid return so the mc can then build the pressure. if a 1-1/8 bore mc at 1/2" stroke can only make 500 psi then what would a 1.25 bore move for volume at .5" stroke. Maybe its not the output of the HB and the pedal ratio not stroking the HB enough.

 

[toddz69]

I'd love to know what pedal ratio is for a factory HB!

On my '01 SD, it's the same as an EB - about 6:1.

Todd Z.

 

[ksagis]

On my '01 SD, it's the same as an EB - about 6:1.

Todd Z.

Do you happen to know how far the pedal moves? Wondering if a different seating seating geometry affects things (I may need to read more of that paper I posted in other thread)

 

[ntsqd]

The caliper had better already be full of brake fluid, that's what bleeding the brakes is supposed to do. If they are not, then we know what the problem is. Let go of the volume concerns, they are not a driving factor and are a distraction. Multiple pumps is not indicative of the m/c not having enough volume. With the line-lock acting as a check-valve multiple pumps tells me that there is something compressible still in the system.

Once the pad retraction/knock-back distance is compressed, the m/c piston really doesn't travel any further. The tiny distance traveled beyond that point is due to flex in system (lines & hoses expanding). Brake fluid is incompressible, once the pad to rotor gaps are removed pushing harder on the pedal doesn't cause the m/c piston to move, it just causes the pressure to increase. Any movement at the pedal past the point of removing the gaps is a lack of rigidity in the system, not more volume being expelled from the m/c.

Domestic OEM m/c's are all designed with roughly the same piston travel. I used to know the exact number, but it's been ~23 years. I want to say that it is between 1.125" and 1.250" In any case that travel is more than twice the worst case piston travel that I've calculated.

 

[toddz69]

Do you happen to know how far the pedal moves? Wondering if a different seating seating geometry affects things (I may need to read more of that paper I posted in other thread)

No idea but I could check it at some point.

Todd Z.

 

[Oldtimer]

. . .I have zero information on the origin /application of this HB or MC. . .

So we are 9 pages deep in this thread.
You have never been happy with the brakes..
Brakes (& steering) need a 100% reliability.

I understand wanting to determine the failure mode, but at this point I would pull every thing and build a system from the brake pedal to the pads/drums that had complete documentation.

 

[ntsqd]

I kind have to agree. Nice to know what's wrong, but it's time to make it work.

Use a later model m/c with a Ø1.25" bore. Bench bleed it before putting it on the truck. It's a PITA and messy, but it makes everything afterwards go much easier.

Honestly, I'd go with the std Ford calipers. I've never been a fan of the T-Bird option. If you need a replacement in BFE that just isn't going to happen. Use an appropriate lubricant on the mating surfaces between the calipers and their mounts. On the advice of a GM brakes seminar lecturer I use silicon dielectric grease there. The calipers need to be able to slide as freely as possible.

Install all new hoses AFTER verifying that they are clear and clean inside.

Leave the line-lock out of the system. Can add it later once the system is functioning as it is supposed to.

Either gravity bleed the system or use a pressure bleeder at the bleed screws to gently (!!) push the fluid uphill into the m/c. Like normal, work from the furthest bleed screw to the nearest bleed screw.

 

[Oldtimer]

And for the H-B?

 

[chuzie]

I understand wanting to determine the failure mode, but at this point I would pull every thing and build a system from the brake pedal to the pads/drums that had complete documentation.

I kind have to agree. Nice to know what's wrong, but it's time to make it work.

As nice as it would be to toss on all new parts to simply make it work, I'd lose sleep at night not knowing what the root cause of the problem was.

Additionally, I love this kind of meticulous mechanical investigation and think all of us will benefit from the shared knowledge this 8 page treasure trove provides.

You guys are freaking awesome and collaborating with you all is a privilege and a pleasure.

 

[ba123]

Might take a bit of fluid, but you could always go part by part starting with the most likely. Or maybe start with the master and new lines and that'll get the fluid loss part done.

 

[chuzie]

Might take a bit of fluid, but you could always go part by part starting with the most likely. Or maybe start with the master and new lines and that'll get the fluid loss part done.

1996 C2500 MC (A/C Delco 18M742) installed and working well.
2000psi at all 4 corners. Bench bled the hell out of it and bled all four corners making a huge mess and going through at least a couple quarts in the process.

Right now I have the front brakes on the forward port and the rear brakes on the aft port. I think this is opposite of how the C2500 was setup, but I'm exhausted and didn't feel like making another new hard line to swap them around again, but it is something I think I'll need to accomplish soon as the reservoir is not 50/50. The forward res is smaller than the aft res and my front calipers are much larger than my rears.

FWIW -the stroke of the HB output rod is appx 0.65"