68Hybrid

Can anyone help me diagnose this brake problem?

Car: 68 El Camino, with LQ4 / 4L80E / 9"

Brakes:
-Fronts are from an SSBC manual drum to power disc conversion kit-- they work fine, no real complaints other than the fact that my calipers are cast iron and 1 piston.

-Rears are the standard "GM Metric Calipers and Rotors" that a lot of ppl use as a cheap rear disc conversion kit.

Master/Booster:
Mid 70's corvette master, 9" booster

Problem:
I have no rear brake pressure.

How this problem arose:
I went to a car show, in the El Camino. On the way home, i burst a brake line. Silly me-- my rubber brake lines in the back were rubbing against my shock absorbers and i didn't realize it. Fortunately, the line burst while i was stopped. I putted through a neighborhood until i was able to determine that i was only losing fluid out of half of my master cylinder. So, i drove the car home on front brakes only and never lost any fluid/pressure to the front brakes.

Note: Up until the day this happened, I put 6,000+ miles on the car with this setup with no issues of any kind.

What i've done:
- replaced and rerouted the brake line(s) i rubbed through.
- bled the system the traditional way.
pedal is soft, feels like air in the lines- performed my test* - fail
- bought a vacuum bleeder and bled the system
pedal feels the same- performed my test* - fail
- bought a pressure bleeder and bled the system
pedal feels the same- performed my test* - fail
- bled the system the traditional way with a friend
pedal feels the same- performed my test* - fail
- replaced master cylinder, bypassed stock distribution block / stock proportioning valve
pedal feels the same- performed my test* - fail
-bought a brake pressure gauge and found that i only have 100-200 psi at my rear calipers.... i've been told i should have AT LEAST 700 psi.

*MY TEST: With the car on jack stands, i start it, put it in drive and let it idle with no wheels on, but with the lugs holding the rotors on tight. Speedo reads 10 mph-ish. Hit the stop pedal as hard as i can. Thus far, best i've gotten is for the rotors to slow significantly, but not even stop... with nothing more than the power of the engine idling in drive.

What am i doing wrong here? Both master cylinders provided the same result... I push the pedal down and it feels as though something bottoms out- like i'm hitting a hard stop, but i'm not. The master cylinder is not bottoming out, because if you hit this "hard stop," hold it, and crack a front bleeder, the pedal will go another 6+ inches to the floor.

I'm pretty confident in my bleeding skills. I've talked to SSBC tech line, Summit tech line, all of my friends who have some experience with this stuff, and finally, Goodies Speed Shop who told me i'm "...out of the realm of common problems," but were otherwise no help.

This problem arose in November-- hasn't been on the road since. Please help me get my Lola back on the road...

babbage1109

iTrader: (11)

Check the line for anything pinched or At the connecting block? Can you check pressure where the two lines come together?

SparkyJJO

iTrader: (11)

Sure sounds like you either have air in the line or a blockage of some sort.

How are the rear line(s) run? One line to the pumpkin, with a rubber hose attaching to the axle, and hard lines running along the axle tubes to hoses at each wheel like a 3channel fbody rear axle, or is there a separate hard line for each side running along the body, with a hose then from the chassis to the caliper?

roy

Is the piston in your combination valve back in the center position?

eb110americana

iTrader: (4)

If you have good pressure at the front, then you have something wrong with the proportioning system. I'm assuming that since you replaced the master cylinder that this is good, and because the system worked prior to the leak that the rear circuit is good too, which only leaves the proportioning system.

If you also have weak pressure at the front brakes, you likely have a problem with engine vacuum or the brake booster itself. You can test this by pushing down the brake pedal with the car off, when you turn the car on (with your foot still on the pedal), it should drop down significantly. You can use a manifold vacuum gauge to easily check manifold pressure on the vacuum lines and to check for leaks. The pedal feel you describe (hard, but drops when a brake bleeder is opened) sounds like loss of power boost. I had this on my car, so I know how crappy it feels.

68Hybrid

@ babbage: no pinches or kinks anywhere. and i need a special fitting in order to put my gauge anywhere except in my calipers. I hope to get that fitting on Monday.

@ sparky: i have a hard line running from the master to a frame rail, then to the center of a crossmember, where it goes to a flex line. Flexline goes to the pumpkin, then splits to go to each wheel. Half way from the pumpkin to the calipers is hard line, then it switches to rubber.

@ roy: this was a concern for me. replacements aren't cheap, nor is getting it rebuilt. plus, i'm not running any of the original brakes that came on the car. So, when someone mentioned that it could be this combination valve, i took it out-- i bypassed it.

@eb110: proportioning system consists of a year old adjustable wilwood unit. what do you think could be wrong?
I've done this booster test you mention, and the pedal sinks about an inch. i thought it would sink more... but, although i haven't put a gauge on the front calipers yet, i'm pretty sure i have adequate pressure up there-- i've taken the car around the block a few times and have the braking power to lock up the fronts. If this could be a booster, I might just replace it anyways. i'm sick of this problem, and want my car back on the road!

Thanks for all the help guys!

eb110americana

iTrader: (4)

If you have an adjustable proportioning valve, is it possible it's adjusted too much towards the front? Can you crank it to favor the rear and see what happens? I know the theory behind the proportioning system, but I can't say I have experience having ever needed to service it.

The pedal test is what kept throwing me off too. Mine would also drop somewhat, so I thought the booster was okay. I brought it to a shop and had the service manager check it out in the parking lot. His experience told him that it wasn't dropping far enough, so I replaced the booster and he was right! Since the booster was only functioning to partial capacity, that is why it was hard to diagnose. Thankfully the shop was willing to offer that bit of advice to me for free and I did the work. Anyway, I'd suggest checking the proportioning valve issues and check for manifold vacuum first. If those both check out, put a new brake booster in.

1ltcap

does this car still have the 60's style proportioning valve down below the master cylinder? the one mounted on the frame rail? they have been known to cause such problems. they shut off the leaking portion of the system when pressure goes away. i've never seen one stick on a gm, but had a few on chryslers and fords. what happens, is that they'll stick in that position, even after you've repaired the problem.

68Hybrid

@1LTCap: I've been told that might have been the problem, so i removed that piece. Right now, 1 line coming from the master splits into 2 for each front caliper. The other line from the master goes through a wilwood adjustable proportioning valve then to the rear axle where it splits and goes to each caliper.

1ltcap

gotcha.

what i'd do then, would be to check pressure to the adjustable valve. see that you have pressure going in. if you do have pressure there, then check each line coming out individually.

the easiest way, would be to have someone sitting in the car, foot on the brake pedal, and use a line wrench to crack the fitting loose. wear safety glasses, just in case it splatters in your face.



it sounds like you kept the braking system pretty simple, so there's not much that it can be.