I got a 2001 ZR2 that had a fair amount of upgrades done to it including rear disc brakes. Everything was fine for a few months since purchased but now I am getting intermittent brake fade.

The brakes work great 90% of the time. Nowadays it seems like if I stop and back up or stop and go forward the pedal goes to the floor and barely stops the rig. I almost hit one of my own other vehicles the first time this happened but luckily I was not going fast. So after the brake fade, If I drive a little, all is well and they work great. Just put on a 2K mile road trip. Also, when they go to the floor and I need them right away again, I can pump them up and they seem to work OK until the next stop or reverse movement.

I am guessing somewhere in the neighborhood of a vacuum issue but not sure where to start. Most of the brake system is all new but not sure about the booster. Supposedly all vacuum lines have been replaced also. Any ideas on getting started on this?

Thanks.

 

Not a vacuum problem, sounds like you have air in the system.

 

So from what I am reading elsewhere, I should do a gravity bleed with no pumping. Correct?

 

yup just watch and keep fluid in master cylinder and if you do try the two person pump style one single pump stroke only. By being able to pump them up and regaining brakes is what makes me wonder about a air bubble in the system.

But however I should have said that if the booster is going out it will usually make a loud whoosing sound as brakes are applied. And normally from the ones I've been around it's not a hit and miss if it goes bad it stays that way.

If there is lack of vacuum brake peddle would be rock hard when applying the brakes.

 

It actually had 4-wheel discs from the factory. Brakes are pretty much all mechanical too, can't think of how a vacuum leak might affect it.

And yeah, it sounds like air in the system. I'd start by doing a conventional bleed just to see what you manage to get out of it just for the sake of knowing what was in there. I'd progress to gravity bleed if problem persisted and/or air was coming out intermittently.

Depending on how bad it's been neglected, a flush might be called for too. In short, brake fluid is hygroscopic and attracts water over time; water boils when it gets hot and becomes steam that compresses when hydraulic fluids aren't supposed to, leading to intermittent fade when the brakes get hot. If you're already doing a 4-wheel bleed anyways this basically only costs you a few more minutes and $6 for a quart of brake fluid, just keep topping off the master and pumping until it runs clear at each wheel.

If you're still getting problems, it's time to consider things like bleeding the ABS unit with a high end 2-way scan tool to initiate the ABS self-bleed process, and of course checking the system for leaks.

 

the same may happen with ruptured flex hoses. It is possible that the outer layer of the rubber hoses to the wheels ruptures and the inner core bulges out like a balloon.

The symptoms could be the same as air in the system.

 

OK, Tried bleeding the brakes. No luck. still have brake fade. Mostly when going slow and backing up. (weird.) Bleeding them was funky. Tried using a brake bleed vacuum pump. Seemed like lots of air coming from RR but could have been the vacuum hose sucking air at the bleed nipple. Pulled 10 oz. of fluid through. Then fluid stopped and vacuum gauge was reading 10" hg but nothing else was coming out. Bled LR and similar except fluid quit coming out but could not obtain a hard vacuum. Bled FR and FL and they bled the normal way. Still had brake fade and had run a total of 16 oz. of fluid through the master. Next day drove back and forth in my driveway and had someone hold the pedal down. Bled the rears and got fluid but did not seem like any air. Did this twice each rear. Still have brake fade.

Next step is to have the ABS module bled (whatever that is) at a shop. Then we'll see. Shop people seem to think it's the master cylinder. Will change that out as a last resort.

 

When my master cylinder went, my brake pedal would push back against me. I would have to literally stand on the pedal for the brakes to engage. Have you tried following the brake lines for any kinks and bends? Are you losing fluid?

 

All flexible brake lines are new. Not losing fluid...

One of the weird things that occurred without me doing anything is in the weeks prior to my bleeding the brakes, the e-brake light quit coming on when I applied that brake. I was experimenting using it to prevent day time head lights and I could not get the lights to stay off. Then after I bled the brakes, the e-brake light started working again and allowed the lites to stay off. Something fishy here. Kinda seems to point to something electronic that is being touchy at least about the e-brake indicator. Either way, I still had the intermittent brake fade. I guess I'll find out after having the ABS looked at. (Damn Gremlins!)

 

Back again.....
Had the lines and the ABS bled at a shop. They said no air came out. I'm guessing at this point that so much new fluid has run through the system that they have basically been flushed. Still have the intermittent brake fade. The shop swears it is the master cylinder but I have heard of people having the same problem and changing the master did not help. Taking a road trip then considering the replacement of the MC upon return...