vicrattlehead

Hello All,
Wondering if a problem I am having has happenned to anyone else. I bought a great '79 about six months ago, and I have been extremely happy with the car, but I have been having a problem with the brakes. The brake pedal has always been very low, and the "Brakes" light is on. This is what I have done to try to solve the problem:

1.) Bled the brakes. Installed all new Raybestos pads. No help.

2.) Took it into the shop (D&M Corvette-Downers Grove, IL). They siad the master cylinder was not doing it's job, and that the front calipers were leaking. I had them replace the master cylinder.
3.) It worked well for a while, but then back to the same old problem. So I replaced the front calipers with new ones from Mid-America Designs. Also installed Stainless Steel Lines up front.

4.) Now it stopped better, but the pedal was still low, and the "brakes" light was still on.

5.) Got a $30 vacuum pump to bleed the brakes better. A little better, but still a low pedal, and brakes light on.

6.) Got "Speed Bleeders" bleeder screws (which are awesome). Pedal is still low, but now the "brakes" light only comes on under a hard brake, then goes off with a quick pedal depression..

The master cylinder is holding a full fluid level on both ends, and there are no leaks around any of the seals. So I don't think I have any more air in the system. But the pedal is just still so low. It has gotten more firm, but I still do not trust this car in an emergency situation. Could it be the booster? Master Cylinder? Hard lines? Pedal travel adjustment? I am at a loss. Please let me know if you have had this problem, and what worked for you. Thank you!
:cry

jerryp58

In between 2 & 3, did the MC lose a lot of fluid and/or empty out? If so, did you bench bleed the MC after installing the new lines & calipers?

Carl in LA

I gotta put this baby back to the top... I just blead out a nice big bubble from my rear driver side caliper and my pedal still goes to the floor... and this with new master cylinder, booster, calipers, discs, spindles, and bearings (I really should of swaped this car)!!!!

vicrattlehead

Hi Jerry,
The master cylinder did not empty out or go real low. In fact, I filled the calipers with as much fluid as I could before I bolted them in so that I would not have that problem. The fluid in the MC was never lower than the bottom edge of the inside lip of the reservoir (about 1/2"). :confused:

mrvette

If your calipers are not leaking, and the front ones are new, get a rear caliper rebuilding kit, and rebuild them leaving the springs out fron behind the pistons....do the front one's that way too, careful on the outer seals to not damage them on removal....
end of pumping problem due to rotor runout, pedal will be slightly softer, but still plenty adequate,....also while you are at it, replace all fluid with DOT5 fluid....expensive but worth it...

GENE

V8yunkie

I would shoot for a bad proportioning valve - the brass type valve underneath the master brake cylinder. This valve is supposed to give that warning light if something is wrong with the brake system - and it also does that, if it is stuck.

Good luck,
Thomas

Top_Notch

Before you go any further, have you checked the runout on all 4 corners?
Was it within spec?

thejaf

When I first bought the 1978, I was taking it to a mechanic for the same problem you're having. After he replaced the most of the brake system, turn out it was a warped wheel hub, and sometime in the past the right rear wheel must have hit a curb a decent speed. Replaced that trailing arm and haven't had a problem with the brakes since. Needless to say, I don't take cars to general mechanics anymore unless it's under warranty.

68427

I've had simular problems and a soft pedal for years. I've also changed out all the parts over that time as well, but nothing helped. Here is a suggestion:

Locate where the problem is. Buy several brass caps and unions to cap off master cylinder and two calipers. Start at the master and cap both lines, bleed the caps and check for a hard pedal. This can be done easily and quickly by one person. Remove one cap, hook up one brake line, bleed brakes - now you have isolated front from rear - check for a hard pedal. Once you have which end is soft (mine was the front), disconnect the calipers, cap lines and bleed. In my case I had a hard pedal again so I knew the master, the bias switch, the rear calipers and all lines were okay.

You may also want to unbolt the calipers, block the pads (I used extra brake pads) and bleed while holding the calipers by hand. Rotate and tap so that any air trapped inside the iron caliper body can make it out the bleeder. This actually works, I found quite a number of air bubbles still inside the calipers after normal bleeding. Hopefully, you are now good.

I wasn't and a second set of suposedly good used calipers didn't work either. So I installed two beutifull Willwood Superlight billet aluminum calipers ($175 for billet or $115 for cast) saved about 15 lbs per wheel - but that's another story. You should be able to get the stock system to work, but caliper design has improved quite a bit in 40 years.

I'll try to post a picture in a couple weeks after I figure out my new digital camera.

Good luck.

Tom454

Good troubleshooting technique... I use it too!
The "capping off" is done by using inverted flare plugs... available only at a few autoparts stores.

In a pinch... you can use a short length of tubing with a flare nut. Just hammer the tubing flat for about six inches and if you hit it hard enough, the line will be sufficiently pinched off to hold the brake pressure. Then you can test per above.

Also, check your brake hoses,... they may be ballooning.