FireballXL5

So... Troubleshooting my newest demon... And trying to search the forum and finding many similar but different issues... And typically once I find a thread with similar symptoms... There's little or no info after that... So, I thought I'd describe MY symptoms and see what direction I should be looking.

The problem began about a week or so ago ... Hopped in the car (81 vette, 383 stroker, my daily driver for a little over a year now) and after warming up, pressed the brake pedal to drop her into gear... The brake pedal felt spongy for the first little bit, inch of travel or so, then felt like it hit a hard spot and firmed up... And activated the brake warning light in the dash cluster. Pump the brake pedal and once it reaches that 'hard spot' it turns off the brake warning light.

I don't typically use the parking brake, but it does turn the brake warning light on and off when I pull and lower the E-Brake... So that system is fine...

Anywhoo... I was at work , and headed home, so after some quick brake checks to verify they would still function and stop the car, I proceeded to diagnose her on the ride home.. Btw, the brakes and pedal pressure have felt just fine for the last year that I've been driving her...

On the road , I can apply the brakes softly, and the car slows down normally... But the pedal travels a little further than I'm used to... 1/2 inch to 1 inch further perhaps... Occasionally , it'll run down to a point where the pedal gets firm(1.5 inches down or so) and then the brake warning light will come on... If I press the brake again through the same range of motion I can get it to go off again... The application of brake pressure still felt even, and wasn't sticky... Slowed the car down normally, but had a different feel at the pedal than I'm used to ..,

Get home and check the master cylinder fluid level and it was slightly low in both reservoirs. Not overly low like it has a leak, but not full and topped off like normal... So I top it off for good measure ... Think it's been 3 months+ since I last checked it... And since filling it , it has not dropped at all in all subsequent test drives since...

As I drove around testing, there came a point where firmly pressing the pedal would no longer reset the brake warning light, and it would stay illuminated all the time... And this entire time , the brake pedal would not ever depress further down than 1.5-2 inches of travel... After some further research I wanted to start my repairs/diagnostics with a full 4 wheel brake bleed..

With help, I went for the two man pump up-open bleeder-close bleeder-repeat method... Per the AIM manual I started with the driver rear, then right rear, and as each bleeder opened, the pedal would drop very minimally... Only about 1/2 inch or so before it stopped dropping... Then when we got to the right front caliper, when the bleeder opened, the pedal dropped ALL THE WAY to the floor! Easily 4-5 inches of brake pedal travel!! The same thing happened with the left front... No air bubbles were noticed to speak of while bleeding either...

But... That didn't solve my problem... I can still get the brake light to come on in the dash when depressing the brake pedal... And it will go off when I press it a second time... And the pedal only travels the same spongy 1 inch before hitting a very firm spot, then builds pressure that doesn't drop or release as I continue to hold foot pressure...

I realize this is getting very long winded... But I wanted to record all symptoms and attempted fixes... I'm going to post this info now, then get into the things I found when playing with the vacuum lines and booster...

FireballXL5

Reserved for the rest of my findings... To be updated soon...

So... To check the vacuum line running I to the booster , I just pulled the hose connected to it while it was running... There is lots of vacuum going into that hose, the motor would stumble and idle rough until I plugged it with my thumb .. Good there... Then later after turning the car off and unplugging it, I can hear a hiss of vacuum escaping... I traced all of the vacuum lines I've got T'd off the rear port(which is a lot) and all seems to be holding pressure..

Mind you, things have all been functioning properly for the last year... But let me give you a run down of what's hooked into this system... Lol.

It's a Holly. 650 double pumper carb... And off the main rear port I've got a T that goes to a pcv valve(which makes little sense cause the other valve cover has a breather and not a sealed filler cap... And I'll get into that in a bit)... But anyways, there's a T, one side runs to the pcv, the other runs towards the brake booster... On it's way to the brake booster there is another T tree that sends two lines up to the front headlights and a smaller set that runs into the vent control switch and a plunger under the dash...

The headlights operate and function as they should, although within the last month or so it's developed a lazy eye... Still works, but slower to respond than the other.. And after this happened, I've noticed that I can no longer turn the car off and get the headlights to lower 'one more time' like it would in the past... I've suspected that the vacuum canister up front may need some attention, and planned to tackle that project later... Don't know if it is all one connected problem... But the brakes acting up recently has become my main focus...

Back to the pcv valve... When the car is running... If I pull the pcv valve and plug the bottom with my thumb, the increase in vacuum pressure allows me to press the brake pedal and the brake warning light will not come on , or definitely not as often... So... For the time being, I have removed the pcv valve from the vacuum system, temporarily added just a second breather to that valve cover, and plugged that line... At an idle, the light cones on less or not at all... At 1500 Rpms, the brake light will come on almost every time I push the pedal and it feels squishy until I hit that hard spot in the pedal at 1.5-2 inches down... But all the while the brakes are functioning...

So, with almost a year of trouble free service... I'm leaning towards a tear or break in a vacuum line, the headlight vacuum canister diaphragm, or the brake booster seal... I'm not thinking it's in the master cylinder or proportioning valve or calipers/hoses... Although those are still possibilities... But, before I go blindly throwing $$$ at new components ... I thought of ask you fine folks what you think...

THANK YOU for taking the time to read my extensively long post here... You guys have helped me in the past, and I've tried to help others... Thus is just looking like a problem that corvette owners will tend to understand a little better than some, and might have the specific knowledge I need to send me in the right direction here... THANKS AGAIN!

FireballXL5

Quick question for the gurus... Will the vacuum system affect the proportioning valve for the brakes?

If the road is wet , and I press the brakes quickly, I can get the front brakes to lock up relatively easily ... Makes me believe the proportioning valve is sticking and routing power to the fronts only... Just guessing...

Is it easier to try and 'clean' the existing valve? Or find and buy a replacement? Or ... Are all my recent issues related to vacuum loss somewhere in the system? Gotta do something... Just hoping to get some insight as to which system to look into first.. ;-)

'75

The problem is in the fluid part of the brakes, not vacuum. Your distribution/proportioning valve is limiting the fluid to the rear brakes. You need to bleed the rear calipers, both bleeders on each.

ezobens

You are either introducing air into the system through the calipers (rotor warp and/or worn caliper lip seals are the biggest offenders) or your master cylinder has an internal leak.

dochorsepower

I believe that what you are referring to as a proportioning valve is the warning distribution block (that may not be the proper name). You will find it under the car on the frame. Its job is to warn you of an imbalance between front and rear. It sounds like there may be something wrong in the rear circuit, rusted up line, air. leaking caliper, stuck caliper or soft expanding flex line. Remember how old these cars are.

FireballXL5

Thanks for the responses... The only part that has me scratching my head is that it was a 'all of a sudden ' issue.... When I drove to work, it was fine... When I tried to leave work, there was a problem...

I did try to bleed the brakes at all 4 wheels... And the one thing that has me feeling like I didn't get it all the first time, is that there was one brake bleeder that didn't want to open... Driver rear outter... So it is possible that the issue with air in the line could be there...

But the fact that it was an immediate issue, not a slowly progressive issue, is trying to tell me something... I'll attempt again to get that bleeder loose without breaking it etc, and give the brake bleeding process one more chance before delving deeper into other options...

ezobens

I had a similar issue with my 69' before I replaced all the calipers with O-ring calipers. I'd bleed the entire car and the brakes would be great for a day or two and then I would get the spongy pedal and the brake light action again- Rinse and repeat... Finally got tired of it and replaced em' all.