HELP! Need help with BRAKES!!

[Steve Reynolds]

OK all you brake experts out there, I have a brake problem that I can’t seem to figure out.


I can’t for the life of me get a solid pedal. If I pump them several times they get marginally better, but still not a good pedal feel. I’ve bleed the brakes about 4-5 times now and there is NO air coming out what so ever. I tapped on all the calipers to help remove residual trapped air. I re-aligned the front solid mount calipers to the rotors and they are now as good as they can be. Even with the 8 to 1 pedal ratio I still feel that the pedal is way too soft. I can’t even hold the car still with the line lock.

Here are the components:

3000lb drag only car, 70 Buick GSX replica, (GM A body).
These are manual brakes with 4 wheel disc.
In the front I have a Wilwood disc brake conversion kit with vented rotors. (This setup is slightly used from a street strip application, hence the vented rotors).


Front - Dynalite Billet 4 piston calipers (Pistons are 1.75” diameter).
Rear – GM Metric calipers (Piston is 2.5” diameter).
Wilwood dual master cylinder with 1” bore.
3/16 brake lines and stainless flex lines.

The system is plumbed with the rear portion of the master going to the front brakes. It goes through a line lock and a proportioning valve. (the proportioning valve is currently wide open).
The front portion of the master feeds the rear calipers.

I have an 8 to 1 pedal ratio.

What else can I try?? Do I have the wrong master bore size?
I posted on several sights so far. There has been suggestions but nothing conclusive.

Please HELP!!
I have Bowling Green coming up soon and I want to bring this new car!

[Smartin]

Might be a dumb question, but did you bench bleed the master?

[Steve Reynolds]

Hi Adam,
Yes, I used two fittings with hose attached to each routing them back into the reservoirs. I slowly worked the pedal until all signs of air were gone.

There are no dumb questions about this problem. I'm taking it back to basics trying to cover all bases, so THANKS for the thought!

Steve

[John Hutchin]

I'm having the same problem with a 71 GS 455. This came on suddenly. I've replaced master cylinder & boost unit, with the same behavior afterwards. Bench bled master cylinder, replaced all fluid in entire system. Still the brake pedal goes way down with some initial resistance. It'll stop the car eventually, but only grabs when pedal is nearly at the floor. I just received a new proportioning valve from Inline Tube last week but haven't had a chance to put it in yet. I hit the lines at the PV with penetrating oil a few times cause I can just see them sticking and twisting off and then I'll be into replacing the lines as well. I'm thinking that's likely the problem (hoping I should say). The car feels like it's braking with the rear wheels only, but I haven't had the nerve to get moving fast enough in it see if they the rears will lock up. If a spring in the proportioning valve broke, I guess it could behave this way.

Let me know if you've already solved your problem ... if not, you;re probably about ready to take a hammer to something. The best fall cruisin' days in the mountains are just about here! Gotta get this fixed!

Huck

[Steve Reynolds]

Thanks Huck,
I switched the front and rear lines and plumbed it the "conventinial way", removed the master and re-bench bled, and used a "power bleeder" to final bleed the system. I also switched back to a stock length brake pedal as mine was almost 8:1 ratio. My brakes are much better but still not "great". I'm off to the track as I type this to give it a try!

It sounds like you have a bad PV. That would be my next step.

Hope it works for you. Hang in there!

Steve

[BillMah52]

Steve
Had a similar problem on a Chevelle tube frame car a few years ago. We checked the system several times. A real head scratcher.
Turned out we had too much braided flex at the calipers which allowed excessive expansion under pressure.
Shortened all four to just what was necessary and improved pedal feel and holding capability dramatically.

[John Hutchin]

I've discovered after talking to the guys at InLine tube that my wife's GS of 18 years apparently had been upgraded to front disks by the prior owner (or someone!). Upon removing the old "proportioning valve", I discovered that it was really just a distribution block, which is used for 4-wheel drum brake setups. Whoever put the disks on didn't know that they should have changed to a proportioning valve to supply more fluid pressure to the disk brakes. It's odd that I've had no problems braking for 18 years. Perhaps it would have only shown up during emergency braking - with the rear locking up and the car spinning out.

I figured now that I have the whole system drained, it's as good a time as any to switch to all stainless lines and the correct proportioning valve.
BTW, here's a picture comparing a distribution block to a proportioning valve for a 71 Skylark/GS

[Steve Reynolds]

Bill, I bet you hit the nail on the head with my problem!! I used the same "very long" front hoses on my other car while experiencing the same issue. This winter I'll order shorter hoses and see if it does the trick.

Huck, looks like you've solved your problem also!


THANKS guys!!

[John Hutchin]

Hmmm... I finally managed to find the time to replace all the brake lines with new stainless ones so that I could put the correct proportioning value in for front disks. What fun. Now, with everything back together I'm STILL having the same problem I started out with!

Any more ideas anyone? I've replaced: Master cylinder/boost unit; proportioning valve; all steel brake lines; brake pad and shoes. What else is left? I guess my next step is to try the longer rod that comes with the booster unit - there are two, a short one and a long one. The previous booster had the short one, so I'm assuming the new one should as well. I don't think the master cylinder will actually fit onto the booster with the long one, but I'm willing to try about anything at this point. No leaks at the rear wheel cylinders or front calipers - although when this whole thing started the front master cylinder reservoir was down about half from the top - so I guess the fluid had to go somewhere.

Proportioning valve needs adjusted? That might be the place to start I guess. I'll have to see if I can get to it - I think they put the adjustment on the front end instead of where you can actually get to it without two elbows on one arm.

Huck

[10sec 455]

Steve,
Did you bleed both upper bleeders on each caliper?

[Steve Reynolds]

Jeff,
Yes I bled all 4 in the proper order. I got the brakes to work good enough to go to Bowling Green. However, after the week of racing the front rotors are warped bad. I found out that when I turned them I didn't remove enough material to work properly. I'll be replacing the rotors with new ones along with shorter front lines, (which I believe is giving me the "soft" pedal).

Huck, I don't know what to tell you about your issue.

Steve

[John Hutchin]

I was wondering if I need to bench bleed the master cylinder again. I did originally, but then ended up replacing the lines and p-valve. The fluid level never went below about 3/4. I guess I'll try that tomorrow when I try to use the longer pin out of the booster to the MCylinder.

Huck

[John Hutchin]

Thought I'd follow-up with the eventual fix. The symptom was a somewhat firm pedal that didn't apply braking until it was about 1/2 to 3/4 to the floor. I could stop the car, but emergency braking was out of the question, since it took time for the pedal to travel to the depth where the brakes would work.

It turns out I had a bad rear wheel cylinder on the driver's side (front disc + rear drums). I didn't see any leaking fluid until I really cleaned everything up and pumped the brakes a lot. Even then, I had to pull the cylinder boot back to see that there was fluid leaking out. The line was sucking air and hence the back brakes were not staying bled. Two $26 wheel cylinders later and all is well. I could have saved myself a lot of work and money by changing those first, but I guess now I won't have to worry about anything in the braking system for another 40 years. I discovered from a mechanic friend of mine that a quick and easy way to help diagnose problems like this is to drive the car and brake repeatedly, then just go around and feel the wheels to see which are hot and which are not. As soon as it was apparent the fronts were doing all the braking the problem was easier to narrow down. That's so simple it's embarassing I didn't think of it!

I'm hoping to make it to Hebron with the family for the Nats! Haven't been to a good Buick event since Norwalk's Buick Race days about 4 or 5 years ago.