brake issues on a vette

[WS6]

Ok guys I am pulling my hair out over this. I have a z07 vette here at the shop that is giving me worlds of trouble with the brakes. The Z07 cars had 13in front brakes. The pedal went soft on this car so I figured the MC was shot internally. It also was pushing very little fluid out the calipers when bleeding. However, with the new remanned MC the issue is still there and actually worse now. I have bench bled this thing twice for a very long time to ensure all the bubbles where gone. When bleeding the rear calipers, the pedal remains what has been considered firm for now with every pump and bleed. Once we went to the front calipers we would get one bleed out of the front and then the pedal was dead. If we let the car sit for a few minutes then come back, the pedal feels back to it's currently normal self. To me this is saying there is air in the system, but no matter how much I bleed no air comes out. I have pressure bled the system and regular old pump a few times then bleed method. Nothing is working to get this pedal firm. What am I missing here? There are no leaks, the pads are brand new, the rotors are with in spec. I am not seeing any issues with the hoses. The car even with the pedal feeling like crap stops fine and will engage ABS. However, the pedal did not feel like this before it went soft which is what prompted me to replace the MC in the first place.

Thanks guys

[MrQuick]

you have air in the system still.

Do a bottle bleed at all four wheels. Then do an ABS bleed. Your scan tool should be able to prompt you on the procedure and bleed order.

[WS6]

I have bench bled the MC for a third time. It immediately went clear when bench bleeding. I bled the back brakes and the MC pushes fluid great and repeatedly. I do one bleed on the front and it goes great but the pedal immediately is dead. Give it a couple minutes and it's good again. We did get some air out but the pedal did not get any better at all. If I pressure bleed the system, no air comes out. I do hear what I think is the MC drawing air in when the pedal is being let out. So I wonder if it is possible to some how draw air in without allowing fluid out because I have no leaks anywhere. I've put so much fluid through this system it is rediculous. I feel if I have air in the system, it's being drawn in some how and not just sitting there waiting to get out.

Oh, and even when the pedal feels "good", turn the car on and it's still crap.

My Snap On Tech 1 says to go out and engage ABS in order to purge the system. What a bunch of crap considering the FSM says to use a tech 1 to do auto bleed. Thanks Snap On. I did go out and engage ABS a few times, but it made no change good or bad.

Thanks Vince.

[MrQuick]

No problem Trey,

Do you have a friend that has a tech 1? that looks like the only way to get a full bleed.

[WS6]

Nope. I have a few more things to try come Monday, but beyond that I don't know what the hell to do.

[WS6]

Vince can you better explain some of this to me. I want to make sure I have it right in my head. How can the ABS work well but still have air in the system? I just drove it again and ABS engages fine, but the pedal is still crap. I would think with air in the system ABS wouldn't work well because it wowouldn't have the power to regulate the fluid correctly. Also, how could I be hearing air coming from the pedal while inside the car and the booster not be hard? I just noticed the air noise a minute ago. It sounds like it is vacuum leaking in through the pedal area. To me that would mean the booster wasn't creating the vacuum it's supposed to and the pedal would be hard. The only other odd thing I have noticed about all this, which aslo leads me to believe air, is that when bledding, the fluid will get sucked back into the caliper when the pedal is released. Shouldn't it be drawing fluid down from the resevoir not back in from the lines? This only happens on the front calipers. The rears are fine and have been fine for sometime now. Thanks.

[MrQuick]

Its real difficult helping diagnose a problem when not being able to see the car. I was only offering suggestions.

The ABS will still operate. The wheel speed sensors and ABS solinoids will still operate with air in the system. The amount of air in the system to cause a soft mushy pedal feel is very little. We both know a scan tool is required for a proper bleed but I've been lucky with a gravidy bleed at the hydraulic control unit. This would also depend on how many lines that come off of it are going up then down.

The woosh noise when you apply the brake is normal. It is the rear section of the booster venting to atmosphere when you push the pedal down. You might hear more noise since the pedal travel is greater with the presence air in the hydraulics.

Im not really understanding the last part of your explanation. The air is most likely in the hydraulic control unit itself. It is very difficult to remove the air without the use of a scan tool. The scan tool will cycle the solinoids and then push a large amount of fluid out the bleeders when you go through the bleeding procedure.

Another thing to remember when bottle bleeding is not to go all the way to the bottom stop on the master cylinder. This can damage the master cylinder seal causing the fluid to internally bleed in the front section of the cylinder.

*EDIT*Lets not forget,you are working with a reman master too. I've experienced many problems with remans.

[WS6]

I appreciate your insight Vince. I'm hoping that the reman unit isn't some how to blame. I'm so lost I am just making sure I have my basic principles correct.

As for my last part, I use a hose off the bleeder to keep the fluid from going everywhere. I have always simply put the end into a cup submerged in fluid. This has allowed me to simply open the bleeder and slowly work the pedal in and out. The fluid would not come back into the drain hose when the pedal was raised. With this car it does and I've never had that issue before. This only happens on the front though.

Thanks Vince. I do appreciate your time

[MrQuick]

yep that is the proper way to bleed it. I use a clear water bottle to make sure the fluid comes out clean.

No problem, any time I can help.
I wish some GM techs would hop in an give some insite. I know you guys are out there.