Interesting brake issue

[jlwdvm]

These brakes on my 69 Firebird track/street car have me a little baffled. Ron Sutton put my complete system together based on what I want to do with the car, weight, tires, etc. Willwood 6 piston in front with 4 piston in the back using Spec 37 rotors, Willwood 7/8 master (already rebuilt by Willwood and bench bled) with Motul 600 fluid.

I initially bled the system via gravity, vacuum, pump and hold. It took a while since the complete system is brand new...including the copper lines that I bent. I then drove the car about 50 miles in hopes that the vibration and bumps from the road would release and residual bubbles still in the system. I then repeated all the bleeding procedures. I then went out to bed the pads in.



I drove the car on a couple occasions and wasn't thrilled with the stopping power (I couldn't get the brakes to lock up if I really wanted to). I was also having trouble with my brake pedal not returning all the way up after being depressed, which caused the brake switch to remain activated...thus the rear brake lights would stay on unless I put my toes under the pedal and pulled it up all the way.

I briefly talked to Ron via email and he suggested to rebleed the system with the rear off the ground a foot or 2, adjust the brake pedal so the master was slightly preloaded, or a combination of the 2.

I rebled the system, and added reverse bleeding to the regiment. I also adjusted the brake pedal rod so it was twisted out far enough to give a slight drag on the brakes while at rest (it is in the upper pedal hole). No air in the pedal as far as I can tell, and the pedal springs all the way up now (no brake switch problem).

Took the car out for a short drive today to test things. I made about 6 hard stops from about 50 and was able to get the rears to lock up (too much rear bias...still a adjusting the proportioning valve. but after about 3 miles of driving the brakes began to lock up! It was so bad that at one point it was like I was power braking the car as I was trying to get to a stop sign and then pull over. I shut things down and got out to check under the hood, under the car, etc. All looked normal, so I got back in and fired it up, but it in gear, and it wouldn't move...brakes completely locked up! I sat there for a bit scratching my head. 2 different cars stopped to see if I needed help...gotta love back country roads in Iowa. I decided to just get in and give it hell. I was able to get moving, but was worried I was going to really mess something up. As I was moving, I did the toe thing with the brake pedal. After I put some pressure on the bottom of the pedal, the brakes seemed to release and all was ok. I applied the brakes twice more on my way back to the garage. I jacked the rear of the car up to see if I could rotate the tires. I could, but it was VERY hard...same on the fronts. After the car sat for a while, the wheels were easier to spin. Sorry for the long post, but I am sitting here wondering "what the heck is going on". I am waiting to hear back from Ron, but he has a few million other things going on at the moment. Anybody have any ideas?

[blklightning1]

Did you re-adjust brake pedal pushrod for approximately .030" clearance/play after the last bleed session? Sounds like master cylinder piston is not releasing completely.

[gator68428]

The good old fashioned two person brake bleeding method gets the air out fast.

And make sure all of your calipers are installed so the bleed screw is at the top (don't see why they wouldn't be...)

My guess is the first time, they weren't bled all the way. After the second round, maybe after bedding in the pads, and with heat, they became way more grippy with the preload.

Now it sounds like they are bled, and bedded in. Get rid of the preload. Never heard of that--doesn't sound like a good idea to me. Then you'll prolly be fine. When in doubt two person bleed is guaranteed.

[a67]

Did you re-adjust brake pedal pushrod for approximately .030" clearance/play after the last bleed session? Sounds like master cylinder piston is not releasing completely.

What is quoted here and add a return spring to the brake pedal. By pre-loading the pedal via the M/C the compensation port is being closed. That is what is causing the brakes to be applied, the fluid is expanding as it heats up.

Bob.

[RobNoLimit]

Yes. The lock up issue is caused by heat. - As it cools, you can then turn the wheels. I would never suggest to pre-load the master. We like to have a very slight lash, or free-play, in the linkage, say .010" to .020". If there is a Booster (not sure) check the fit from the output rod in the booster to the piston in the master. - Most are too long and cause a pre-load of the master. The link to the pedal should have a light free-play. If it does not return, start with new nylon pedal bushings, graphite grease, and then if needed a light return spring. Sad to say, but Iron GM masters have less temp swell problems than cool looking aftermarket units. Last, if temperature increase was the cause of the lock up, there may still be some air in there. :(, bleed it again. Let us know what fixes it. Good Luck

[jlwdvm]

Thanks for the info! The consensus on here and Lateral-g is that the preload on the master allowed pressure to build in the system and then with the added heat=locked brakes. I am in the process of adjusting the master rod...again. What is considered free play? I have been looking at it as the amount of movement in the pedal from rest to where I can't push it in anymore using one finger on the pedal. Maybe I am looking at it wrong. Also trying to visualize how a return spring would be mounted to keep the brake light switch working properly. Thanks for all the help!