Fading brake pedal......again.

[AUSSIEVETTEMAN]

High guys I am stumped on this one.

We have replaced most of the brake parts on our 81 Coupe, the latest is a rekit of the master cylinder but we have not put it in yet I'll give you the chain of events first.
Here is our problem:
While travelling up to a mates house on Saturday night to pick him and his Vette up for our annual Corvette Concourse (he has lost his licence), I noticed our brakes starting to fade. By the time we got to his house the pedal was on the floor. We disconnected the booster line and it let all the pressure go, then took the cap off the master cylinder to find that there was hot gas coming from the master cylinder reservoir and the fluid overflowed from the master cylinder. It was not overfilled last time it was checked.
Ok, no brainer, we had air getting into the brake hydraulics, possibly indicating the seals in the master cylinder were cactus.
We managed to crack the lines and bleed the system enough for me to get the old girl home. By the time we got there the brakes had started to fade again and we were looking at leaving the old girl home for the convention the next day.
That is when me mate called two other guys from the Corvette Club, father & son, and he brought his master cylinder from his 77 around. We popped the hood on our 81 and took the cap off the master cylinder, again the gas and the overflowing. We applied the brake pedal and two squirts of brake fluid came up through the reservoir. OK we defineately had air!
Out came my master cylinder and in went his, his has been rekitted and resleeved. We fitted the MS and bleed the brakes. I took it for a test pilot around the block and the first brake aplication nearly put me through the windscreen! I had never felt the Vettes brakes like this before and I liked it.

Well it has been 6 days since we did the midnight repairs and in that time I have rekitted our master cylinder and it is ready to go in.
Trouble is that I lifted the bonnet yesterday after the Vette came home from the tranny shop (see our other post) and found that the fluid has been leaking from my mates master cylinder and has eaten the painted surface he had on it away! I disconnected the booster again and it hissed at me, then took the cap off and found the heated brake fluid gas escaping and slight overflowing, but I had the rags ready.

Problem now: My mates master cylinder needs a new paint before it goes back in his 77. It looks like I have another problem booster related (it is rebuilt) and looks like I need to repair his master cylinder with a rekit before I give it back.
What do you think is happening?

Cheers

Tony

 

[Vettehead Mikey]

I don't quite understand why you're disconnecting the booster to bleed 'pressure'-it has no effect on this problem. Keep in mind that the booster works under a partial vacuum, not a pressure. If it was some how implicated in this issue, then it would be sucking out the fluid, not pumping in a vapour or a fluid.

It's completely normal for a booster to retain vacuum after the engine is shut off, that's what the check valve at the end of the hose is for.

The two jets of fluid you see in the reservoir when you pump the pedal are normal.

My guess is that you have air entering at the caliper piston seals, (rotor run out due to improper lathe turning?) but have no idea why your M/C appears to have 'gas' in it other than the air that's entering the calipers is pushing the fluid backwards to the M/C making it 'appear' to be over filled.

 

[AUSSIEVETTEMAN]

Mikey,

We disconnect the booster line to get it out of the way to get the MC cover clips moved out of the way. The cover is hard to get off without removing the booster hose as the clip comes back and get in the way of the lid.

Ok, so the two squirts are normal, thought I had major dramas there.
The calipers have been re-kitted, there are new rubber lines on each caliper, the pads are new all round and the front has new rotors. The booster and now the master cylinder are now both renewed, but I seem to have the same problem with my mates MS, so the air entering via the piston seal idea is a real possibility. That would explain the gradual decline in pedal pressure over the last couple of days.

Thanks Mikey, looks like the caliper piston kit is faulty on one wheel and the air is coming in from there, trouble is which one. All have been rebuilt, needle in a haystack time.

Cheers

Tony

 

[Vettehead Mikey]

It might be that the new rotors that you fitted are giving you problems with lateral run out, causing the pads to be knocked back into the caliper. This action can allow air to get sucked past the lip seals on the pistons and the resulting soggy brake pedal.

GM never intended that the rotors be separated in the field from the hubs, so took no special care in machining the mating flange between the hub and rotor to be square and perpendicular to the axis of rotation. They originally riveted the two pieces together, and machined them as a set with virtually no run out afterwards.

When the original rotor is replaced with another, there's no guarantee that the minimal lateral runout spec has been maintained. You will need to have this measured with a dial indicator and corrected if much over .005" or so. The bubba way of fixing things is to put shims between the rotor and hub, but the proper way is to have the rotor and hub machined as a set.

Next time I'm visiting my companies' office in Brisbane I'll give you a hand with it!

 

[AUSSIEVETTEMAN]

Thanks Mikey,

The fridge is always stocked and the BBQ is always on the ready.

So, the best way is to machine these puppies is on the car. The rear are still original and are riveted on the spindle, the fronts can come of but that defeats the purpose as the hubs will still be on the car. The fronts are still new and technically shouldn't be warped, but may be enough to be out of tolerance and are acting like an air pump. I'll get a mobile machine guy around to skim off the rotors on the car.

Mental note for next time: machine rotors and hubs together.

Thanks again Mikey.


Tony

 

[AUSSIEVETTEMAN]

Thanks Alan,

I took the wheels off and spun the rotors around and didn't see a lot of warping, but it doesn't mean it is not there. The wheel bearings are new, both front and rear. Could the rear do the same? They are original and are still rivited to the spindle.

Cheers

Tony

 

[Vettehead Mikey]

Tony, The wobble in question is mere thousands of an inch and .005" cannot usually be seen by the naked eye.

It's too late to tell you to put the originals back on, the only remedy is to have them measured and machined either in situ, or as a matched set on a lathe.

I doubt very much if your rear rotors need to be touched and encourage you not to replace them unless there is no choice.

 

[AUSSIEVETTEMAN]

Mikey & Alan,

OK guys, to double check what good info you gave, I re-bled the entire system and drove her to work for a week. The brakes were fine, but had at that point started to fade again. By Wednesday the following week I defineately noticed the difference and the peddle was starting to get spongy and then by Friday, it was sponge right to the floor.
She still pulls up, but compared to the inital peddle I had after bleeding, there is a big difference.

I went out and purchased a dial indicator and set her up on the drivers side rotor (was passengers side in the USA).
OK, the braking system is still in the factory position and has not been changed since she had her RHD steering conversion in 1996. The booster and Master Cylinder are still on the left.
Anyway back to the dial indicator. I set it up on 'zero' and spun the rotor slowly with two lug nuts holding the rotor to the hub. The result is a difference between -.002 and +.005. I believe that the run out is the problem.
Q: Would the run out pump that much air in to the master cylinder or line and overfill the master cylinder with the cap off?

I rang a mobile machine guy who does on-car machining. He said he can machine both the hub and rotor together off the car. He said his gear could not machine it on the car as the hub is too big. Fair enough, at least he can do the hub and rotor together.

Looks like you guys are right. It is skim time!

Cheers

Tony

 

[AUSSIEVETTEMAN]

Alan,

Thanks for your reply.
I had the front rotors and hubs machine as a unit by bolting them together with the wheel nuts. When I installed them last Friday arvo, I checked them again and found that the left side had 0.001" and the right, 0.003 within factory specs.
I ran out of time and decided to bleed the system on Saturday morning. When I jacked the rear of the Vette up and removed the wheels, I decided to check the run out on the rear rotors. Mmmm....the left side had 0.015" and the right had 0.021". Looks like I found the problem, so I bled the system anyway to buy me some 'brake time' and booked her in with a local mechanic. I told him to double check the rear wheel bearing setup as it was disturbed when the rear wheel studs were replaced. I asked him to do an 'on-car' machine and not to separate the rivited rotor from the assembly or if he did, to machine as a unit. He set his machine up to 0 tolerance and machined the rotors to within 0.00075" on the left and 0.001" on the right, give the old girl a quick bleed and I picked her up that day.
So far so good. I have peddle half way down the travel of the peddle (boosted). The peddle is a little different, it didn't seem pulsating before but it must have been as she pulls up smooth now. I could never get it within factory anyway as there is a rod going from the right side to the left to control the brake system. Although she has had a right hand drive conversion, the braking system is still factory. Better that way parts wise and also because there is limited room on the right with a steering box living under custom made headers!
I could not see any fluid leaks on any calipers, but I will keep an eye out now.

Have you had any feedback on your Urethane diff mount. My transmission guru has just replaced my flex plate and torque converter (instructed by me as I have a harmonic type balance issue at between 2700 and 3200 rpm). I installed a urethane diff mount, not your VBP brand, and he believes it may be a little harsh for the driveline when you back off from the gas and the driveline geometry changes. I have your 'cattle dog' (catalogue) here, so I'll check what you have got as I am thinking of going back to a rubber mount on the diff only. The poly suspension can stay as it is great, no probs at all, the best thing I have done so far apart from the engine refresh and kicking off that Holley.

Thanks again,

Tony

 

[AUSSIEVETTEMAN]

Nah, no good the peddle has gone to the floor again after three weeks.

I got back under her this arvo and checked the lines from master cylinder to the rear and also checked the front lines as well. No cracks, but there is a small crease in the line where it goes under the input drive shaft on the cross member. I got someone to pump the peddle and I had a good look around at the lines, no evidence of leaks anywhere. I ripped the wheels off and checked the calipers for wet spots, all dry as a bone.
I checked the calipers for operation and had someone hold their foot on the peddle and checked for caliper movement, all OK. I bled the brakes again and came up with a solid peddle about 1/3 depressed before it firms up. I kicked the old girl over and pumped the peddle while the engine was running, but the peddle nearly went to the floor. I shut the engine off and pumped the peddle back up again about 2 or 3 times and it went rock hard again.

I don't know guys, I am starting to get frustrated with it. The booster is new, the master cylinder is new, the calipers have been rekitted with new seals, new pads, new rubber lines front and rear, new rotors on the front that have been machined with their hubs to within factory specs, the rear discs have been machined oncar and are within factory specs.
I thought maybe I am not getting all the air from the lines or maybe one of the seals on one of the calipers is letting air in. Starting to question the booster now as well, the engine is a little bit more improved now and maybe it is effecting the booster or the hose is not right.

I am pulling the rest of me hair out!

Cheers

Tony

 

[JohnZ]

If there are no leaks and the pedal goes to the floor, it's the master cylinder; if you had air in the system, you couldn't pump it up with the pedal.

 

[AUSSIEVETTEMAN]

If there are no leaks and the pedal goes to the floor, it's the master cylinder; if you had air in the system, you couldn't pump it up with the pedal.

Thanks John, I have had the master cylinder rebuilt just recently. While it was being rebuilt, I borrowed one from a fellow Club member's 77 and had the same result.
Any thoughts?
I am thinking of getting one of the pressure bleeders you mentioned in another post.

Cheers

Tony

 

[00fxd]

My experience is, it's always the rear rotor run out. If the rotors rivets have been drilled out it is often possible to rotate the rotor on the studs to achieve acceptable runout. It is said that the rotors were riveted for ease of assembly at the plant. Go ahead and drill them out.