I'm trying to diagnose a symptom here. What I've got is the pedal goes all the way to the floor, quite easily. And if I'm sitting there revving up the engine, the pedal will get stuck on the floor for a second and slowly come back up. If I'm not revving it, and just let it idle, it doesn't get stuck on the floor. The brake pedal doesn't have a hard "feel" so I don't think it's a vacuum problem but I'm open to ideas.


Here's what I have on the car and checked/replaced:

Freshly rebuilt 4th gen fbody front and rear disc brakes
Brand new 4th gen master cylinder
Checked to make sure the booster is working
Brand new braided front brake lines


Does this just sound like there's air in the lines somewhere? I bled the system twice, maybe I wasn't thorough enough considering I had the lines and calipers off the car most of the winter while rebuilding everything.

 

Yes, you most likely have air in the lines, and your master cylinder may have an issue with the return/piston springs or you have an issue with the booster where the atmospheric pressure air inlet valve gets stuck open. Did you install a check valve to the booster? With the engine revving up, you are pulling less vacuum then when the engine is at idle, which leads me to believe that when you let pressure off the pedal and the master cylinder return springs are supposed to force the pedal rod back and close the atmospheric air valve/open both sides of the diaphram to engine vacuum, one side of the diaphram is not getting engine vacuum or is still getting atmosphereic pressure.

 

You sorta lost me towards the end there. I know that the booster holds vacuum as it should from testing it. I ran the car for a bit, shut it off and then pulled the hose off the booster and it sucked air in. The master cylinder is a brand new part but I haven't messed with the proportioning valve at all.

Can you explain what I can test to figure out the supposed vacuum issue? I'm bleeding the lines again hopefully tonight to get rid issue of the pedal going to the floor with little or no resistance, but it still doesn't sort out why the pedal gets stuck on the floor temporarily.

EDIT:
I did bleed the hydraulics tonight and the pedal seems to be a lot better, should have stopping power now. But it's still doing the same thing with the pedal sticking to the floor under throttle. I disconnected the vacuum line from the check valve going into the booster and the pedal firmed up real hard and it did NOT get stuck on the floor with the line disconnected. I'm not sure if that helps in figuring out what's wrong...

 

So the brake booster works by applying engine vacuum to one side of a diaphram, while allowing atmospheric pressure in on the other side. If the side that has atmosphereic pressure applied to it does not recieve vacuum or continues to recieve outside air pressure because of a faulty valve, the pressure will hold the pedal down. When you are at higher RPMs, the vacuum signal is weaker, making a faulty valve even more probematic to equalizing vacuum on both sides of the diaphram. Your booster probably needs to be replaced or repaired.

 

Well, I'm definitely thinking it is an issue in the vacuum system somewhere but from what I've been reading online, I'm not 100% sold on the booster being the problem. The reason I say that is because the brake pedal doesn't have a hard feel to it, it's soft. If the booster was bad, the pedal should be very firm to press down on, and it's not. I am going to do one more check tomorrow by pumping up the brakes and holding the pedal down, then start the car and see if the pedal softens up. It should if the booster is still good.

If it's not the booster what else could the problem be? I'm getting so frustrated it's not even funny.

 

Bump

 

Im thinking you still have air trapped in the master cylinder. Bleeding the brakes will not cure this becasue of the shape of the MC bolted to the booster. The air remains trapped. You need to bench bleed it at an angle to get all the traped air out and then install it onto the car.

 

Cool I'll give that a try. I'm not exactly sure how to bench bleed a MC but a friend of my dad's has a shop and said he can try that for me.