reverse brake bleeding didn't solve pedal fade problem

[Big Blocker]

Eddie,

If you can't find a plug, buy a short piece of tube with the right ends and make a loop at the "T" in the rear . . . brake pressures aren't all that high.

Doc

Correction to the "that high" comment:
Yes, brake pressures are in the 1K psi range, when you have a square inch (or more) to work against.
I was relating the comment to the 1/8" line that will be plugged for Eddies' test.
Sorry for the confusion . . .
Thanks Jeff for bringing this to my attention, for clarification.

[Gopher]

Are you using a 2 LBS residual pressure valve in the rear brake line. The valve will keep the rear pads from walking back. Don't know if this will help but the tech guys at Wilwood told me to use one when I was talking to them about a spacing problem on my car.

[kenny832]

Eddie, need me to come back over? I can get away Thursday night.

[JKleiner]

Originally Posted by Big Blocker

. . . brake pressures aren't all that high.

I don't know what you consider "that high" Doc but I think 1,000 psi + is getting up there!

Jeff

[Mustang Man]

Residual pressure valves are generally not needed unless the master cylinder is lower than the calipers/wheel cylinders, as in an under-floor mount setup. Common on street rods, not so common on our cars.

Mark

[NiceGuyEddie]

Hey Mark:

Okay "generally not needed" - bet you are right, but if it helps, i'd try it.

but first, i like Pete's idea of making a loop from a "T".

the place where the hard line goes to the soft line to the rear is high up. if i disconnect the soft line, i am assuming the air bubble will be up high, and it won't affect what's going on with the front brakes? the reason i ask is because it will then be impossible to bleed.

[ClemsonS197]

Eddie,

I was chasing a soft pedal for a while. So I pulled my master and bench bled it again. I watched a youtube video on bench bleeding and one guy mentioned that it might take 30 minutes to get the air out. For some reason that stuck with me.

So I started to bench bleed it, pushing on the master with a screw driver and the air kept coming out of the rear port. About 10 minutes later, the air seemed to be purged. So I let it sit. 15 minutes later I went back and decided to do 100 more pushes just to be sure. On push 80 something, a giant air bubble appeared. After that, no air. Waiting again, and about half way through another giant air bubble, then nothing. After I thought it was completely bled, I kept getting a single, large air bubble. I must have bench bled that master for an hour and thousands of pushes. Every 50 to 100, a single air bubble would appear. I wonder if you have a similar issue since it's not repeatable but randomly occurs. That could be your single weak pedal effort, then good brakes after that.

[NiceGuyEddie]

i recently took a very twisty drive up to the mountains... the ones where there are lots of "S" turns at about 30mph. when going thru these turns the pedal was almost constantly faded. i had to tap the brakes every 20 seconds to keep the pedal up. don't worry, after that tap, the brakes are solid and i was safe. cruising downhill in neutral riding the brake for 30 seconds at a time, the pedal was always solid. let up on it, go thru a turn or two (20 mph) and the pedal would be faded.

i'll still do the "block off the rear brakes" thing but for the moment it's pointing away from the master. (i hope)

[FFR5452]

Eddie, if it were me, I would take it to a qualified mechanic and pay a pro to determine what the problem is. Then have him fix it.

You're playing with fire here by driving winding mountain roads (or anywhere for that matter) with that brake problem. Just think of this scenario: You are enjoying your favorite winding mountain road (with your girlfriend du jour in the pasenger seat) when you round a corner and are faced with a boulder (animal, oncoming vehicle, drifting motorcyclist, etc.) immediately in your lane and you have to lock em up right now. You hit the pedal and it goes to the floor. No time to pump the pedal before impact.

I know you're a problem solver, but brake problems should be attended to immediately. We don't want to be reading about another Cobra flying off the road.

[NiceGuyEddie]

don't worry, Jim. i was very careful, no hot-dogging, and it's just a quick pump and the brakes are stiff as a rock. my '73 Dodge Dart was worse! the problem isn't that bad, maybe should not have even said anything.

i plan to block off the rear brakes, and if the problem is isolated to rear axle slop, i am going to bring the car to the guy that did my rear end gear swap for a full rebuild.

.

[CraigS]

While applauding Jim for his concern w/ safety, I don't think you can go to any regular mechanic and get this fixed. I spent the last 20 yrs as a Lexus tech and can tell you we would eventually get it fixed and, where I worked at least, you would only pay for the actual fix. But where would you find a tech/shop like that who is qualified to work on an FFR? Do the rear block off test by getting someone to weld up a plug similiar to the one you made w/ epoxy (no way that will hold). BTW, when you install that plug you will need to bleed the system. I suspect that will be a two man job. One guy pumps the brakes and then holds pressure on the pedal. Second guy loosens the plug somewhat so fluid/air can escape around it. Tighten the plug and then yell to man one to pump the brakes again. This may take longer than regular bleeding since you don't actually open a passage for the fluid to flow out of. It just has to leak around and through the threads of your plug.Good luck.

[Big Blocker]

Eddie,

If you make a small loop of brake line and install it at the "T" on the rear housing, you shouldn't have any issues with "bleeding the short loop by loosening one of the fittings and getting someone to press on the brake pedal - slowly. That will push brake fluid thru the loop and bleed out the loosened fitting to a point where the loop is now filled with fluid. Tighten the fitting and test at the brake pedal for firmness.

You should be able to drive the car now (being mindful that you only have fronts to stop you) and see if your issue goes away, it should, if my assumptions are correct in that you have a disk pad push-back problem due to rear axle free-play and created/compounded by the fact that you have "solid-mounted" calipers.

If your calipers were mounted in a "floating" condition, we wouldn't be having this thread . . .

As an alternate thought, if you don't want to build a loop, buy two pipe plugs that are the right size and thread and install them in the ports for the left and right rear brakes (again, at the rear "T").

HTH

Doc

[MPTech]

Eddie, what was the resolution to this problem?

I just started having a brake fade problem (was fine last week).

[SeanHM]

You might see if kickback springs are available for your rear calipers. We run em on the race cars. I want to say 7 lb if memory serves. The piston on the AP's have a nub on the inside that the spring locates on. I'm not familiar with the Wilwood internals but I 'd think that is pretty common amongst the manufacturers. Gordon could probably answer that. After seeing your later posts, it doesn't sound like the issues I have had with mc's before. I have often gotten a good pedal before only to have it fell mushy a few minutes later until I did a pressure bleed and even in some cases a hot bleed. The prototypes usually just needed the pressure, but the GT3 required a hot bleed to get a good pedal. It's not uncommon to bleed brakes 4-5 times on a race weekend. Cheapest thing to try anyway.

[NiceGuyEddie]

hi dudes

i have not had any time to work on the roadster - not even close. i haven't even had the time to wash it!

i had the car out this past weekend, about 60 miles of mixed driving. the pedal was only pre-faded about twice. the problem was not nearly as bad as it was when it was at its worst. who knows, maybe there really is a bubble in there that is slowly working its way out?



i definitely plan to do the "T" thing after the new year.

[ryans88gt]

I had the exact same problem when I upgraded rear brakes to non floating strange 4 piston pro race brakes.



It would take 1 pedal pump and then the brakes would firm up.

The issue was simply with 4 pistons the factory fox MC wasn't moving enough volume to engage the pads after they retracted. Runout on the rear axles makes it worse.

I ended up putting an SVO MC in the car (1 1/8" bore), and the pedal was firm after that.

Strange also told me a residual pressure valve in the rear line would fix the problem and that it was a common issue with non floating rear calipers on a solid rear axle with a large volume caliper.

[NiceGuyEddie]

hey Ryan: good stuff, but i think the Breeze Cobr@ M/C is a proven setup on our cars.

i am very intrigued by the residual pressure valve.

http://www.wilwood.com/Search/PartNo...mdesc=pressure

Wilwood two pound residual pressure valves retain a minimum brake line pressure to help eliminate excessive pedal travel in both disc and drum brake systems. The two pound valve is used in disc brake applications where the master cylinder is mounted below the horizontal plane of the calipers and fluid drain back occurs from gravity and vibration, thereby causing excessive caliper piston retraction and a longer brake pedal stroke. The minimal two pound residual pressure prevents fluid from flowing back without causing the brakes to drag. Residual Pressure Valves are made from billet aluminum and color coded for easy identification.

could this really be the answer? not sure about the master cylinder below the horizontal plane thing, but for $20 could it hurt to try it?

would i need one valve before the "T" or one on each side after the "T"?

[Big Blocker]

Eddie,

Your master cylinder is NOT mounted below your calipers unless you have a floor mounted pedal box . . . and you don't.

The 2 Lb. residual valve might be your "fix" for your rear brake issue, it's worth a try. You would need just one residual valve in the line going to the rear. It can be mounted anywhere in that path . . . up by the master (optimal), just before the "T" as a second choice. I'd rather see the whole line kept at 2 pounds, then just the last 5 feet of line. The makers of the residual valve will have install suggestions. You will need to bleed the rears again after installing this "fix". Get the valve, get out your flaring tool and get to it . . .

quote { maybe there really is a bubble in there that is slowly working its way out? }
You better hope not, air leaking out means you have a leak somewhere in the system that will eventually leak brake fluid.

Doc

[NiceGuyEddie]

looks like i need 260-3278, it comes with the two fittings and it says it will work with 3/16" hard brake lines.

i can easily try the "T" blockoff thing and install the residual pressure valve in the same session.

i can't even remember if i purchased my own brake flaring tool.



.

[Big Blocker]

Eddie,

If you're still planning on doing the "block off the rears and see if I have good brake pedal feel" thing, then do that first, to verify it's the rears that are causing your issue. If the pedal still needs to be "pumped" to get a good solid feel with the rears blocked off, you have other issues. But we've been thru all that before . . . it's the rears.

Once you have verified the rears are your problem, then remove the "T" loop (or plug(s)) and install the residual pressure valve, re-bleed the rears and go for a run. I'm betting that your problem will be corrected.

Good luck . . .

Doc

[ryans88gt]

I would guess that would solve your problem.

Also autozone rents brake flaring tools.

[NiceGuyEddie]

i don't expect i can use my reverse bleeding tool on the rear brakes if i have the valve installed.

[ryans88gt]

Originally Posted by NiceGuyEddie

i don't expect i can use my reverse bleeding tool on the rear brakes if i have the valve installed.

Never used a reverse bleeding tool, so that's a good question. The valve will keep 2 PSI in the line and caliper. So if your reverse bleeding tool can't overcome 2 PSI, then probably not.

[Mark Reynolds]

I have not read all the above posts but I feel like I should throw this out there since you mentioned you had two of the master cylinders do the same thing.

Master Cylinder Installation Notes Breeze Automotive LLC
10/15/08

1994 Cobra Master Cylinder/Adjustable Brake Pushrod Failure Mode and how to avoid (detailed)

Over the past couple of years we have noticed a slightly high rate of customer reported “leaking” of new 1994 Cobra master cylinders. We have found that if the pushrod is misaligned near the limit of travel it can contact the circlip, and push the circlip into the master cylinder bore. The result can be damage to the bore and master cylinder that leaks slowly into the footbox area. The circlip groove is tapered on the far side to enable the piston seals to slide in during master cylinder assembly. This makes it relatively easy to inadvertently push the circlip into the bore. The circlip is also susceptible to bending if contacted by any part of the pushrod. A bent circlip can prevent the master cylinder from retracting fully leading to brakes that don’t fully release.

Misalignment of the pushrod to the master cylinder bore is due to variations in the positioning of the modified pushrod pin location on the brake pedal and the condition and mounting position of the donor pedal box. Since the pushrod end moves in an arc where it is attached to the pedal arm the alignment varies during pedal travel. The alignment will not be perfect but it is OK as long as the pushrod never touches the circlip. Pay special attention to the adjustable pushrods have a jam nut that can catch, bend and displace the circlip if it happens to be aligned with the loop ends of the circlip. This is most likely to happen during initial brake bleeding procedures when the pushrod is getting fully depressed. Once the brakes are successfully bled the pushrod won’t normally travel in as far.

If your pushrod contacts the circlip you must reposition the footbox, the master cylinder, or both, and or re-work the brake pedal arm to change the position of the pushrod.

[ryans88gt]

Good info there ^^

[NiceGuyEddie]

there was serious misalignment with my Mustang donor setup, but i will double check everything when i get under there.

very good info on how the jam nut can hit the circlip on the master cylinder during initial bleeding, when the pedal goes all the way down.

.

[NiceGuyEddie]

...slight update:

i ordered the residual pressure valve. given the way my lines are run, it is going to be super-easy to install.

i still plan to totally block off the rears before trying the valve, but i wonder, will driving with the parking brake pulled up just a wee keep pressure on the brakes and do the same thing?

of course, i can always just go out and try it, but i'm trying to understand the mechanics here.

cheers,

[CraigS]

Don't drive it w/ the park brake pulled slightly. The problem is that the park brake is a real crude mechanism meant to apply the brake fully. It works fine for that. But an attempt to run it partially exposes the problem. At least one of the pads will be dragging on the rotor. This heats it up, but you don't notice anything until it's too late. You could get away w/ it for a few miles but that probably won't prove anything so I don't think it's worth the potential problems.

[NiceGuyEddie]

super slight update:

they talked about these pressure valves today on Horsepower TV. they discussed the exact intended application, where the master cylinder is lower than the brake system in an older style car. a nice little diddy to watch if you have the time.

can anybody answer my question about reverse bleeding with the valve? i can "suck bleed" with the Phoenix tool, but it's a bit more cumbersome and messy.

i have not had time to work on the car but i plan to hit it in the next few days.

cheers,

[kenny832]

Let me know when you need assistance; I just installed new header gaskets so I need to stretch her legs out, and I think it's suppose to warm up a bit in the next few days. No more 30's at night.

Originally Posted by NiceGuyEddie

super slight update:

they talked about these pressure valves today on Horsepower TV. they discussed the exact intended application, where the master cylinder is lower than the brake system in an older style car. a nice little diddy to watch if you have the time.

can anybody answer my question about reverse bleeding with the valve? i can "suck bleed" with the Phoenix tool, but it's a bit more cumbersome and messy.

i have not had time to work on the car but i plan to hit it in the next few days.

cheers,