ajrothm

Hey guys,

I have a weird brake problem going on with my manual brakes and looking for some tips. Here is the history.

Back in September, my brakes started going to mush....I checked all the calipers, all looked ok, no leaks etc so I assumed it was the master cylinder. So I put a rebuilt MC on it and it worked great although the cap kept leaking on the MC due to a porous condition on the sealing surface, obvious from previous rust and sandblasting caused the pitting etc. It came from O-reilys. So I go back to O Reily and bought a brand new MC(not rebuilt). I put it on, pressure bleed the brakes and it was all good. I did notice the pedal did not feel quite as stiff as before but I decided to let it ride as it would still stop the ok and lock up the brakes if hit hard.

So I park the car and don't drive it for about 3 wks, I hop in it and take off and the brake pedal is kind of mushy and eventually going to the floor. I put the pressure bleeder back on it, bleed em out and the pedal was back to mediocre and I drove the car two days and noticed the system was getting mushy again. So I park the car for about 2 wks and went to work on it today. I put the pressure bleeder on it and bled it out, I could get pedal pressure but not much and for some reason, I kept getting air in the calipers, I would do RR the LR and back and forth and kept getting air in the system. Finally my dad and I decide to change the calipers, he just received a new set for our 67' so I stole them and slapped them on the 71. We also put on new brake hoses and new steel lines on the rear. Put on the pressure bleeder and guess what? no pedal pressure and air all in the system. Even after I bleed each caliper and it seems the air is out, we can manually bleed them again by pushing and holding the brake pedal and cracking a line and we still get air....We never get decent pedal pressure.

Finally this evening, I just ordered another new mastercylinder from another company, this master cylinder will be a Raybestos and I will be installing and bleeding in the morning. Anything I am missing on this? I have never had a problem bleeding this car's system before, I am hoping I just have a dud MC on it now but......We checked all the lines and proportioning valve for leaks, all is well... WTF am I doing wrong?


Sorry for this extremely long post but I figured the back ground info will help you gurus.....Thanks in advance!!

elevatordude

Check or change the flex line from the caliper it might have swelled and youll have a slight blockage

ajrothm

Yep replaced all 4 hoses today as well as the steel lines on the rear.

Thanks.

elevatordude

how about the roter it might be running out and the pulseing causes it to suck air

LYLE

When you replace the M/C make sure the rod is not pushing the piston down with the bolts tight.

ajrothm

Yes I thought about rotor run out but I can not even get enough pedal pressure to test drive the car. I have not been spinning the rotors over during the bleeding process so they can not be pushing the pads in. It SHOULD be building up pressure. The rear rotors are still original and riveted on and are in EXCELLENT shape believe it or not. I am not sure if they have been replaced and re-riveted or not but they are definetly riveted. All the rotors are in great condition with absolutely no grooves or lines in them.

I am really leaning towards the MC but....... I guess I will find out in the morning. I have been searching the board and found a tip on pinching off the brake hoses and if it builds pedal pressure, I know its air in the calipers. I will likely try that tomorrow.

Keep tips coming fellas!

427-390

Rotor run out. Corvette Enthusiast had an article in this months issue on how rotor run out affects braking. Particularly the mushy brake pedal that you are experiencing after a total overhaul. Article was authored by JohnZ (John Hinckley). You should check into it. Good luck!

elevatordude

have you tryed a motive pressure bleeder I had a hell of a time trying to bleed mine and its the only thing that got all the air out

7T1vette

Have you looked for "new" leaks at the calipers and hoses [since doing the repairs]. Sometimes working on part of an older brake system will cause another "weak link" to fail. If no leaks are noticed, the choices are 1) you didn't get the system bled fully; 2) the "new" M/C at the local parts store (made in Mexico or China) was defective. I'm betting that the Raybestos M/C and a good bleed job will solve your problem.

ajrothm

Yeah I did check for new leaks but.....didn't find any. I just put the new calipers, new hoses and new lines on today. I will have the new MC in the morning.

Yes I am using a Motive pressure bleeder as well as manual bleeding afterwards. Normally the Motive gets it done on its own but......

If its not the MC and its not the calipers, what the hell else could it be?

I have not touched the proportioning valve or anything else and there is no noticeable leaks on any of the steel lines or fittings.

1stC3Vette

There has always been a lot of discussion regarding bleeding techniques for these car using pressure bleeders and so on. Since you replaced the master cylinder, did you bench bleed it first?

Assuming you did, I would open the bleeders at all 2 rear calipers and gravity bleed the system. Once the rears were complete, I did the passenger side front, then drivers side. After that, I did the simple pump the brakes and crack the bleeders method. My pedal is firm as can be for the last 2 years.

Air is either still trapped in the system or you are drawing it in somewhere.

Good luck,
Dino

Z-man

I agree with 1stC3. If you are correct about no leaks, it's your bleeding technique. Do it the old fashioned way or use speed bleeders. They should be rock solid the minute you're done and stay that way - no mediocre anything...

Muddywaters

I had the same problems you are having...I went as far as doing a total restore and still had the same problem one year later after the restore. After atempting to troubleshoot the problem (on this forum) I called a Dave Herrllnger (Corvette frame off mech.) whom rebuilt my TA's and diff. He said after I bench bled the MC...to let gravity do the work for you. Open RR outside, and crack. Let bleed for approx. 20 min....after 10 min. or so...your gonna see bubbles apearing and then finally disapear. Keep and eye on the MC!!! Do not let it run dry!!!! Close RR and open RR inner and do the same. Proceed to LR outer, then inner...then RF and finally LF...believe me, you'll get a very hard peddle Gravity bleeding works!!! Muddy

RunningMan373

check your full car steel brake lines for a coroded pin hole. if OK, this is how I bled the rears on my '73. Set up pressure bleeder, use about 15 PSI. get two bottles and two hoses, put a little fluid in them. set the rear of the car on stands, both tires off. set up both rear inners to bleed, and open both sides at once. Pull in on the inner brake pads **once**, then close the rear bleeders, and step on the pedal to re-seat the pads. re-open and repeat. Move to the out side bleeders. repeat the sequence, re-seating the pads after moving them. re-Bleed inner 1/2 a cup, then outer half a cup, each side. that should do it. takes about 2 quarts of fluid. Be sure to bench bleed the new M/C before install for about 5 mins,, to No air.

LYLE

Their is no way rotor runout is the problem when the car has not been driven and you have air in the lines after the rebuild and it's still setting in the shop. Make sure the length of the rod is not pushing on the piston as I found this to be the problem of air in the lines. I got this tip off the forum after a few weeks and three master cylinders.

ajrothm

Thanks guys, I am going to pick up the new MC, now....I will bench bleed the hell out of it, then put it on, use the Motive bleeder at 15 psi initially and bleed the system in the proper orientation. That USUALLY works for me. If not, I will gravity bleed the rears first, I have never tried that and see how it does. For some reason, I believe air is getting back into the system. No matter how much I bled yesterday, I still got air in it. I am hoping the MC was sucking in air some how....

Can you gravity bleed the fronts also? I would assume you can. I will report back later with results, hopefully good. I am out on christmas vaca and plan on driving the crap outta the vette this next week.

wombvette

The first thing you did wrong was change the Master Cylinder. In 35 years of working on these cars, I have only seen a very few bad M/Cs but I have seen hundreds changed for no good reason. The majority of those changed did not solve the original problem and resulted in a worse condition that I had to fix for them.

You may have two problems here. I would go ahead and change to the New M/C, bleed that out good, and then work on my brake bleeding proceedure. Once you have the fluid flowing in the calipers, just let them sit over night. Then without touching anything, crack each bleeder one at a time to get the rest of the air out.

Muddywaters

Absolutely Use the gravity technique... and you'll certainly be driving this weekend Muddy

Derby98

I recently replaced the mc on my 71 as it was bad. I bleed the brakes with the 'ole 2 man method & did it without success for 1 hour. Had soft pedal.
went out & purchased all new speed bleeders, installed 'em & bled the brakes again & had a nice firm pedal in 10 minutes.
The old bleeders were all coroded & i guess were not sealing tight when the bleeder valve was closed.

LYLE

I have used the gravity method of bleeding with great results. On resleeved master cylinders which some are form the local parts store they will sometimes not gravity bleed at all. I was told the sleeve may cover a hole that will not allow gravity bleeding.