I really hate to post and ask another stupid brake question. I have worked on and installed complete brake systems for more than 50 years now, and have never run across this problem. The system consists of 4 wheel discs with GM metric calipers. Underfloor Corvette MC. 2# RPV's. The rear calipers are the ones with the built-in emergency brakes. About a month ago I swapped out my 8" for a 9" Ford rear end. I already had the discs and calipers on the 8" rear and just reused them on the 9". So exactly the same system that I was having no trouble with previously. I had a little trouble getting all the air out, but ended up with a firm pedal with no pump up. I have driven it a couple hundred miles and have had no problems, but last night coming down the road to my driveway I hit the brakes, the car slowed enough to make the turn. There was something in the drive so I stayed on the brakes and the pedal went clear to the floor. One quick pump and the pedal was back up to the top and I could not push it through to the floor. This also happened about 3-4 days ago. It is a come and go thing. Right now it has good pedal, no pump up, and won't go to the floor. I'm not losing fluid anywhere. I have two thoughts. One is that I missed some air somewhere in the system. I don't think this is likely as evidenced by the firm pedal with no pump up. The other is that a seal in the MC is "breaking over" then flopping back in place??? Or something like that? About gotta be a bad MC doesn't it? I have another MC on the shelf, but before I tear into it and start replacing parts, I wanted to see what you guys think.
If it is a by-passing seal in the MC, the way I have found to test them is: pump up the pedal and hold it with a good bit of pressure. Now, start backing off on the pressure, but don't take all the foot pressure away. If you have a bypassing seal, the pedal will start to drop as you ease off the pressure. A way to tell if there is a pocket of air somewhere is to remove the MC cap. You may want to place some towels around the MC as this can get messy. Pump the pedal and hold it with as much pressure as you can manage. Let up on the pedal and you should see a swell of fluid returning to the MC through the compensating ports. If you have trapped air, the return of fluid will shoot up real high possibly out of the uncapped reservoir.
This is how I've tested for by-passing master cylinder seals for years. Your problem sounds like a bad master, this test will likely confirm it.
If you decide to change the MC be sure to bench bleed it before install…save a lot of bleeding later. We used to bleed with a hand vacuum pump instead of the old way…worked very well on disc brakes.
I'll have to go out and check that. It sounds just like the situation that caused the pedal to go to the floor. Thanks.
Odd thing about it being the MC is (assuming) you just disconnected the rear calipers and/or line at the rear, and moved it all over to the new rear. Assuming that, I’d be looking for air/leak in the rear of the car. I guess though if you’re like me and this afternoon project turned in to a few months, maybe the MC just sat and developed an issue.
The problem may have been caused when the OP bled the brakes. You can push the piston past its normal old range of travel into the "dirty" part of the bore and get some of whatever contaminant is there between the cup and the bore. If the pedal goes all the way to the floor I would check and make sure the MC piston bottoms out before the pedal hits the floor.
Jaw22w , It’s not a stupid question,,,,,and ,,,,,look at the bright side,,,,,,at least you spelled brake correctly ! Tommy
Never a problem asking - brakes are an essential part before and comes before horsepower by a few letters !
I'm trying to learn here...not start a fuss. I ain't clear about the above statement. ^^^^^^ "Start backing off on the pressure"? Where?? You got the pedal pumped up and holding it..."Now ,start backing off on the pressure" How?......you got your foot pumping up the pressure?........Where you "backing off on the pressure" from?? Lil hep here! 6sally6
I’ll try this , the easiest to explain to me is , simulate sitting at a traffic light . You apply lots of pressure to stop and you ease up to hold the car at the light . As you ease of with just enough pedal pressure to hold the car still , the pedal will creep to the floor and your car will start moving . As it does this you start the process all over . Heavy pedal pressure seems to expand the cup in the MC and it works fine , as the pressure lightens up , fluid takes the path of less resistance , past the MC cup back to the MC instead of the brakes . I hope this helps , I tried best as I could to explain . As others have said , I think I would start with the MC , and move ahead with the issue . To me brakes are more important , than going fast .
I've experienced this " low pressure pedal creep" before , always figured it was m/c fluid bypass , never has manifested itself as a significant problem ( to me) so I'm interested to see where this goes . never have had the " go to the floor" thing though , without a corresponding system failure ???
If the pedal is creeping toward the floor,,,or goes straight down,,,,,there is a sealing issue somewhere. This is a hydraulic system,,,,,,even by relaxing pressure somewhat,,,,,,it should still hold steady . If you apply 100 pounds of pressure or 2000 pounds,,,,,the brakes should not let anything roll after a stop . Sounds like a leaking M C to me ,,,,I’ve had it before ,,,,,the seals let it pass internally,,,and the pedal would ease down . Or possibly a wheel caliper is leaking,,,,,however,,,,that will show a fluid trail somewhere . If they are leaking,,,,,the fluid won’t come back,,,,,and your reservoir will get lower as well . You said that it went straight to the floor,,,,and then did it again a few days later. I really believe you have a M C seal about to fail,,,,,,be careful,,,,,it might be giving you a warning,,,,,so take heed . Tommy
Well I tried the hard pressure then reduce pressure test and could not get the pedal to go to the floor. I guess I shouldn't say go to the floor. The pedal doesn't go to the floor. The MC bottoms out very close to the floor but not on it. It is braking good. But I now have no confidence in it. This problem simply must be in the MC. I guess I will be swapping out MC's today. I just love bleeding brakes, NOT. Working on brakes sometimes breaks your heart. (That was just for you, Desoto291Hemi.)
Bypassing masters can be hard to diagnose. Some will do it time after time, others have a mind of their own. Sitting still, we would try to "tickle" the brake pedal into making the master bypass on command. Just apply a light on-off pressure, within a few minutes I could get a suspect one to drop like a stone.
I had a one ton truck that did this same thing. I changed the master cylinder. it did it again the next day. Turns out the outer wheel bearing on the front right was bad,real bad,no noise was heard during the test drive. but when tuning hard left while braking the right caliper would push the rotor way over and the pedal would go to the floor. I guess the caliper held the rotor straight up while driving straight ahead? check your front wheel bearings,no kidding.
I thought it was possible that someone wouldn't understand as I wrote it. You back off your foot pressure on the brake pedal.
Yeah! Changed out MC. All is good. That damn MC was only 2 years old. I put an old MC in it that I had removed from another car.
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