I would like some of you gentlemen's thoughts on a brake issue,
Ive learned long ago that if you start out asking questions before diving head first off the proverbial dock,
into what might not be a simple problem or repair,
and that you generally benefit from listening to and questioning other people,
(theres ALWAYS some guy who knows more than you do on any one subject,
or at least has had much more extensive experience,
and knows a bit because he screwed up or paid a great deal of cash solving some issue)
as they may have spent a good deal of time and money solving an issue and gaining experience you might not have thought of dealing with previously.

I went to bleed the brakes on my 1996 corvette last evening, and the front brakes worked out ok, not so with the rear brakes, so my first though was the master cylinder might be defective as the corvette has sat un-used for a couple months, I loosened the lines on the master cylinder as a test and had one of my grand kids stomp on the brakes,
and the master cylinder produces flow and pressure as the lines exit the master cylinder,
and while the brake fluid was not all that pristine in appearance it works ok on the front brakes so I replaced all the fluid and bled those front brakes,
I don't think its a blocked hydraulic line or defective caliper, in the rear, simply because BOTH rear brakes are acting suspiciously like the pressurized fluid is not reaching the calipers, so the next more likely source of the problem is the anti-lock brakes controller, located behind the rear drivers seat.
so thoughts and any tips or testing ideas are rather welcome?
before I start looking into this.

 

If the flew lines are old replace them. They can swell on the inside and restrict the flow.

 

The ABS on a 95-96 Corvette is not an integrated system. Its hydraulic lines are completely isolated from the main brake system until an ABS event activates the solenoids that connects it to the hydraulic circuits. It is not likely to be your culprit.

 

There's a specific order to follow when bleeding brakes, at least for my '87. Starting with the rear passenger, IIRC.
Check FSM?

 

yes Im well aware of the proper order and its been followed, I appreciate that must be done correctly.
Im just curious why both rear brakes suddenly won,t bleed?
once I find out Ill post what I find / learn

 

Need to get #1 to bleed clean before moving on.

How are you bleeding? I tried a vacuum bleeder on mine, it was junk. Fell back to good old turn-stomp-turn.
Happen to be painting my calipers, off car, and will be installing Speed Bleeders so I can do by myself this time.

 

I don’t know if the brake lines are run differently from 1991 vs yours. Someone here will surely know. If they are different, then my below discussion is likely wrong. I am just trying to help with some ideas.

If you pressurize this closed fluid system, then create an opening in the system, surely, the fluid will blow out. Even if there is air in the system, the air will blow out. Logic says there must be some blockage to the two rear caliper bleed ports. You say when you pressurize the system, then open either rear bleed screw, nothing comes out.

On my 91, the two rear brakes share one line coming out of the ABS manifold. The front brakes have dedicated lines from the ABS to each caliper.

There is a tee on the single line from the ABS manifold behind the driver’s side rear tire. One feed line enters then splits to each rear wheel caliper. With tire off, the tee is bolted to the inner fender at the 11 o’clock position.

I would try to “bleed” the rear system from that connection tee to see if there is gunk blocking at that spot.

Of course, your system may be more advanced than the 91, so the piping may be completely different.

 

Motive Power Bleeder



And replace the flexible brake lines

 

Can you disconnect the caliper and see if it even comes out through that hose? If not, disconnect the hose and keep going back.

 

Ready for the EASY way? I do this (and manage it) for a living. I get paid salary, so I want to get it done the fastest, easiest way possible and move on to other things. I used to deal with this same issue on '90's Phords when I worked at Sunday River as the GV Fleet mechanic and I taught myself this method for getting stubborn systems flowing good, FAST, and by myself.

Close the front bleeders, open the rears. Get in the car and depress the brake pedal to the floor. That will force and contaminates through the lines, and clear them for flow. When you release the brake pedal, yes, you will draw some air into the rear calipers, but that's fine....wait about 15-30 seconds and the calipers will fill back up and begin dripping fluid from the gravity bleeding process.

If it's really stubborn, you may only clear one side, and not the other. If that occurs close the bleeder on the side that cleared...step on the brake pedal again, to blow out the other side....then let it gravity fill/bleed out that caliper. Once they're both flowing, commence your typical bleed procedure to replace all the fluid and don't forget to depress the caliper pistons to help get them throughly flushed, too.

 

..... The likely culprit here is the pressure differential shuttle valve built into the engine side of the master cylinder ... unscrew the plastic switch on that side of the MC and look into the tiny hole that the stem of the switch came out of ... you should see a solid object at the bottom ... if you see any part of a groove in that solid shuttle valve (it will be toward the front) ... the shuttle has moved to block the flow of fluid to the "leaking" rear brake circuit ... you must re-center the shuttle before you will be able to bleed the brakes ... it doesn't help matters that there is also a rear brake bias spring in the same cavity as the differential shuttle that limits the amount of pressure allowed back there to keep the rear brakes from locking up under hard braking ... per the FSM you need to achieve 400psi brake fluid pressure on both circuits to re-center the valve ... not so easy to do when there is air in the system and this is also where that bias spring comes back to haunt you ... I have a very TINY pick that I use to push the shuttle valve toward the front of the car ... its tedious and not easy to do but it works ... after you center the shuttle (it won't /can't) go all the way forward because of the pick but when you get it far enough forward do not stomp on the pedal til you're done bleeding ... push the pedal gently but firmly until you feel resistance then stop ... I use Russel Speed Bleeders because you will only get your wife/GF or kids to help you do this just one time ... push gently until there are NO bubbles of any size in the bleeder hose ... Good Luck !

 

Your '91 uses a completely and utterly different ABS system from a 96.

 

I figured the systems were different with the later years being much more advanced. I was trying to spitball and get some brain juices flowing!

 

Hi
if you have ever had air get into the abs system you need to cycle the abs pump to bleed the trapped air out, the rear brakes have less fluid going to them compared to front due to the smaller pistons and proportioning springs in master cylinder.

 

That's a negative, ghost rider.

 

Its not easy to do without the tool. You have to cycle the ABS pump by engaging it several times at least. I gave up on it after 5 hard stops and pulled out the tool for my Excursion which pulsed the ABS system.

 

His problem isn't likely the ABS, so lets not confuse the guy anymore than we already have, but dwelling on it.

He needs to blast the crud through the system by pumping it out. The fastest, easiest way to do that is to use the pump that's already in the car; the master cylinder.

 

Fair enough. IDK how much crud there is since I flush every 2 years but is there that much that can do this or is the hose collapsed? Maybe remove it at the hose to the steel line and see if that works? I have a bad feeling that the hose is collapsed.

 

thank you gentlemen
I may not have solved the issue yet but the infos helpful and Im getting closer

 

I don't think we care about your flushing habits, but the OP said these clues:

I've seen this many-a-time. Smeg in the brake system inhibits flow to or through some calipers. There is a SUPER fast, easy way to push the smeg through and out. It doesn't involve using a scan tool. If he does this fast, free thing....and can't push fluid through the rear brakes, THEN we might start looking at other components.



.