How y'all doing? I'm new here, as with many other sites, and am really hoping to find someone who can figure this out. The sooner the better, I may have authorized my mechanic to do something stupid.

I've got moody brakes, sometimes they work sometimes they don't this is pedal pump to pedal pump, not day to day.

It's an 84 Z28, 4 wheel disc. So far new calipers and pads have been put on, problem got worse, then bleeding, still the same, found a leak on the combo valve, disontinued part replaced with a used one. No change!!!

Replaced the booster and re replaced the right side caliper (pulling), no change. Went back to the shop for a line flushing, no change.

As we speak, they have blocked off individual lines with one rebuilt master cylinder, and then again with a new not rebuilt master cylinder. NO CHANGE!! Any ideas, please help!!!!
Thanks
New guy
Mark (pics & info to follow)

 

the main issue that happens with the brakes is deterioration of the rubber flex hoses at the wheels and from the body to the rear axle. They deteriorate from the inside and can form a flap internally that will block fluid randomly.

If these lines are original, replace them and see what happens.

the rear calipers are not know to work well on the early cars. They just suck.

by "not work", what exactly happens? pedal drops to the floor? some of the brakes work, others dont? really hard pedal and only a small amount of braking power?

 

i highly agree. i had that problem with a gmc truck. when you hit the brakes they would stay locked. someone had put power steering fluid in the master cylinder. it caused the flex hoses to rot internally.

 

It's a very soft pedal, sometimes it can be pumped up, other times there's nothing there at all. It's as if there's air getting into the system, we've established that there isn't. To go one further on the ???, I've kinda talked the mech. involved to try it out with a prop valve off of fullsize p/u, then I located a used prop off of an 86 z28, neither is the right application, what would y'all do?

 

I also agree on the rubber brake lines. Mine looked fine on the outside but once you pressed on the brake they would cause the wheel to lock at the caliper in my case - the brake fluid does not move very much in the lines to apply the brakes - if one of these hoses was split or bulging inside it could allow the brake fluid to fill that and not the calipers causing a soft pedal. SS flex lines can improve pedal feel as well since they are even less prone to expanding under pressure.

 

I'm already neck deep in $$$ so I'll get him to do that, just per chance it's the prop valve (as is thought), does the truck idea make sense, I figured the weight distribution should be about the same. I'm hoping y'all are right, I'm somewhere between burnt out and obsessed, I've spent thousands to make her go, I can't believe she can't stop.

 

Don't think the port valve is gonna help - it controls the pressure between the front and rear but overall you still have the same pressure in the system. When the valve is not right you can get a hard pedal but no brakes or a hard pedal and grabby brakes depending in where the most pressure went. If the rear gets too much you'll be sidways when you step too hard on the brakes and if the front gets too much you will eat your pads fast but the car prob will stop just fine. Sounds like you have changed most everything so take a step back and look for the obvious - it's got to be something that you would not expect to happen. Remove the rear line from the master and plug it and see if the pedal feels any diifferent - if no try the front one and reconnect the rear - still no different then it's before the lines (master, booster etc) if it does change (worse) then look in that circuit for the issue.

 

Per the mech, they've done this. That's where the prop valve theorum came in. Sometimes there's brakes, sometimes there's not. Oddly you mentioned with a bad valve, both scenarios the pedal is hard, 2 year olds give firmer handshakes than my pedal. I'm getting a raw spot from scratching my head, it matches the lump from pounding it on the wall. 2 different boosters no change, 3 different masters no change, 2 sets of calipers no change, 2 different prop valves (one had a crack) no change. I'll rerun this idea by him, but I'm kinda leaning toward the brake line idea, what's another $100 for no change and they're the only part that hasn't been touched. Love the fact I gotta work and pay extra for all this. Another dumb question, if the lines were deteriorated from the inside, is it safe to assume that the system wouldn't be fully bled? Sorry I think in spurts.

 

I hear ya - been there done that too! Yeah if the lines are blocked or otherwise compromised you might see it when they are bled - if you open 1 bleeder valve brake fluid should come out and continue to drip as long as the top is off the master and it has fluid. Gravity will pull it out - if you push the pedal to the floor it should put out a steady stream of brake fluid and go easily through its travel. You can do this on each front and 1 of the rears (there is only 1 rubber line in the back) and see if anything funny is going on. Tell your mech. to do it the hard way with the pedal and someone watching and not to use a bleeder suction device as this could mask the issue. Also if you are paying someone to do this for you (a business) I don't think you should be paying for them to figure out your problem if they have already quoted you a price. Brakes are not that complicated and you have already replaced the most complicated parts with no resolution. Just my

 

Thanks again for the input, I think we got it. I mentioned your above post, and it was confidently answered that they did plug the front and rear individually, I misunderstood the results. I've got brakes roughly half the time, they're not holding pressure. I left work and hit a few boneyards, (one of which was full of s##t and had me drive 2 counties out of my way for nuthin I won't mention names, Osceola Auto Salvage, oops) but I picked up another prop valve out of an 86, $10 bucks got nuthin more to lose, if this doesn't work I'm bringin her home and doin' the lines and an aftermarket prop. And yeah it does suck having a shop do this, but all the work in to this point, along with the fact that my apartment manager is not chewin me a new one, again, it's worth the buck eighty he's charging. In all actuality, these guys are doing a helluva job as far as I'm concerned, it's just frustrating knowing with facilities I could be doin it.

 

Like the man said get rid of all the old rubber brake lines.

 

Brake lines go in tomorrow, maybe tonight based on beer consumption. Fingers crossed. Thanks again for all advice, I'm really hoping you all are right, seriously, no cheap shot, no sarcasm, I mean it, I JUST WANT TO STOP!

 

I've worked in garages before and anytime we changed the calipers we changed the rubber lines also unless they were new already . The symptoms you explained led me to believe it is in a rubber line like others have said . I have had the old lines blow apart on me from being old . The lines should be changed everytime you change calipers or if they show signs of cracking or swelling .

 

Sorry it's been awhile, just wanted to update. The lines are done, I still have no brakes, actually, I do, right as the pedal hits the floor. System is fully bled. I've had one other mech look at it, he wouldn't touch it, then gave in and towed it acrosse town to a buddy who runs a GMC shop. His shop has had it for thr last 10 days, they have no idea. Here's what they threw at me, they've ordered the master and prop valve for an 88 w/ 4 discs. My ??, has anyone tried this and what obstacles are coming up? Also, any other ideas leaning toward the existing set up?

Thanks again
This is killing me

 

Wow - this is really out there - when the brakes are near the floor are the brakes at all 4 tires on hard or just barely grabbing? Brake fluid does not compress so if you are moving the pedal that much and the master is really moving that much fluid then either a caliper piston is moving (missing pad or installed wrong? at 1 of them or something loose?) or the master / portioning valve is leaking internal. On rear drum cars what you are seeing would usually mean that the rear brakes are gone..pedal goes to the floor but you still have front brakes as a safety so you can stop the car.

Side thought - when the master was changed, was it bled before it was installed?

 

Ok got it. It is done, the master, prop valve and rear calipers off of an 88 GTA. That did it. Thanks again to all for contributing.

 

hey - great to hear it's all set. Throw that stuff away or bury it so it does not get used again. Now you can and not worry about smashing into anything when you need to stop.