A while back I posted a thread talking about brakes and what my problem was. Stiff pedal when engine is off, soft pedal when engine is on. Since then this is everything I have done.

Bench bled the MC again

Check seals on booster and MC with carb cleaner to see any bubbles, no bubbles everything was good.

I disconnected vacuum lines to see what manual brakes I have and then what I had with the booster. Can confidently say that the booster works.

Bled the car while it was running and the rear end was jacked up

Bled the car while the engine was off

Adjusted the rod inside the booster to the correct length

Went over lines to check, but didn’t spot any leaks or any wet spots

Pushed the button on the proportional valve, it's the stock one so fairly old. I took the handle part of a screwdriver and pushed as hard as I could on the button. I didn't hear or feal any clicks or noises.

(For those that didn’t see my last post)

Replaced booster 6 months ago, replaced hard lines a little over a year ago, MC replaced 6 months ago, two out of the four rubber lines have been replaced and the other two are in good condition, gravity bled, bench bled, pump method bled, and close to everything else under the sun.

One of the calipers looked like it was trying to split apart when I was pushing the brakes. I went ahead and tighten the bolts that hold the caliper together but I don’t if anyone else has encountered that so I’m just putting that out there. (Delco brake calipers)

The brakes did not improve after all of this, I have brakes but they haven’t changed any. I have a video of the brake pedal and what it looks like, If you want to see a video shoot me a message and I can show you.

My next move is to buy a motive speed bleeder and go from there and then maybe replace the proportional valve. Any other recommendations?

 

I have a 77. Many times I have done brake work over the years. Replaced the right rear caliper back in about 1990 with stainless sleeved unit. No issues bleeding brakes. Replaced master cylinder in about 95. No issues bleeding brakes. Replaced the other 3 calipers in about 98. And switched over to dot 5 at this time. No issues bleeding brakes. Replaced every brake line and Junction block on the car in 2007. No issues bleeding brakes. Rebuild the right rear caliper again in 2019. No issues bleeding brakes. Replaced all the flex lines I installed in 2007 with stainless braided lines in the last month. No issues bleeding brakes. Brake test after replacing all flex lines. 50 MPH. nailed brakes hard. All 4 locked up. Smoke filled the car. Car pulled up perfectly straight. A week later on the local track. Car pulled up hard and straight.
I'm thinking your flex lines that look perfect may not be. Air trapped in master? Air trapped somewhere?
Get a pressure bleeder!

 

Oh, and a 76 doesn't have a proportional valve. It does have a brake pressure switch that will move to one side if you lose pressure on one line or the other. It has a brake warning switch in it. And the way to recenter it is to stomp on the brake pedal hard a few times. But button? What button?

 

I don't know if your car has a check valve in the vacuum line going to the booster, but if it does, it may be installed backwards or defective. Jerry

 

I went through all the headaches of brakes that you are or have experienced. I was giving up and was just going to let the car sit in the garage. Broke down and bought the Motive Bleeder and now my brakes work as good as they did when I got the car in 1974. Spend the $$ and get the bleeder, and use a big clamp to secure the top onto the master cylinder. The parts supplied with the bleeder don't work well and you may end up with a lot of brake fluid dripping in the engine bay. Good luck.
Duane

 

I thought it was called a proportional valve but yes I've looked into it. My brake light doesn't come on and I tested the switch on top to see if its gets power, by using the tool where you ground it with a clip on one end and then touch the wires on the switch to see if the light comes on in the dash. The light did thus proving to me that the switch worked. I have also slammed on the brakes many times just to make sure. On mine, there is a small black button on the rear end of the pressure switch. Ive also bench bled the MC two different times and I pump it until it has no air coming out. The only confusing part is when Im going about 40 and I stomp on it the passenger front tire locks up and leaves a tire mark behind me, so I know that side is good. Do you think I should still bleed the brakes while the car is running?

 

Enjoy your new Vette.

 

Yes, the switch sits on tops of it, for some reason mine looks different then anyone's I've seen. It has two mounting bolts on the top instead of the top right and bottom left. I don't have a picture nor when I look it up I cant seem to find what my mine looks like. With the measuring tool, when i took the MC to bench bleed it I stuck a bolt in and measure how far it went in and then copied that on the adjustable rod.

 

Can you post a picture of your PV?

 

Just replace the last 2 hoses. It's cheap after all the work you've done. I had one bad hose (on another vehicle) that prevented appropriate bleeding.... I found it hard to believe that just one hose would do that. It will be well worth the cost and your trouble to just replace them.

Also - I cannot find that you said you used a pressure bleeder? If not then either get one, or make one.

I don't even bother bleeding my 68 without a pressure bleeder. It's a waste of time.

 

I used stainless steel braided teflon hoses as the rubber hose can swell under pressure or hard braking.

 

This step "Bled the car while it was running and the rear end was jacked up"

Not sure of the step above. But usually rear end jacked up is to bleed the MC while on the car. This doesn't open any bleeders on the calipers. Doesn't require you to run the engine. Requires you to open the MC cover only and slowly push on brake pedal to allow air in the MC exit via the bleeder holes in the MC.
You need to also check the bleeder holes in the MC are not plugged or closed. Requires a thin wire pushed into the bleeder holes. Also can be tested by a quick stab of the brakes and watch a push of brake fluid fountain stream via the MC. (This step can be a mess)

Not sure if this is what you did here. ??? My post on this step.
Get the last bit of air out from the brake master cylinder. - CorvetteForum - Chevrolet Corvette Forum Discussion

 

The Stainless hoses that attach to the calipers do make a much better feel. The life expectancy of those hoses according to the NTSB is 15 years or something close to that.

IF you do buy a set of stainless steel brake hoses be sure they have a D.O.T. Sticker or number affixed to them. china is making lots of inferior grade hoses and they don't have or will pass the original testing

 

A couple of ideas for you. First, I had the same issue as you...namely working brakes but soft pedal with too much travel. Sometimes taking it out for a short, slow drive can dislodge a stubborn air pocket via the road vibration so if you can do it safely, get it out for short drive then re-bleed using a power bleeder. Note on the Motive - don't put any fluid in it. Just pump it up with air to about 10-15lbs. At 15lbs, let it just sit there for 5-10min. If there are any leaks you'll see them from the pressure. And I use two big gun clamps on the MC cover that comes in the kit. Don't use the chains, they don't work. Lastly, that MC rod length is critical to getting a good stiff pedal. Even a difference of 1/8" off will do it. Make sure the rod is set properly. Finally, after all that you still have a soft pedal, then try and isolate where in the system the issue is. Get a new short brake line that fits the front and rear MC outlets. Cut it in half then smash down the open end really good to create a seal. See pic. Use these to help isolate where there may be air in the system (either the front lines or rear). Insert this into the front MC outlet, start the car, and press on the pedal. If the pedal is now stiff and the travel is good, you know the problem is in the front lines. If the pedal is still soft, re-install the front brake line and re-bleed. Then install your line blocker you just made onto the rear MC outlet. Start the car and test again. If the pedal is now hard, your issue is somewhere in the rear lines (or the distribution block). If it's still soft after this, then pull both the front and rear lines and insert both blockers. Now start the car and test again. The pedal should be rock hard as you've removed both the front and rear brake system from the equation. IF it's still soft with both the front and rear blocked off, you have a MC issue and you should replace it.



Hope that helps and good luck!

 

I've always used the gravity bleed method, draining about 8 ozs. from each caliper while keeping an eye on the mc and refilling as necessary . Never had any problems with air in the system. However, it is time consuming.

 

The 'GAP' between the booster push rod and the master cylinder is critical and should be between 1/32" and 1/16"

In the HELP section of many auto parts stores have master cylinder bleed kits that contain the plugs.
The size you need is:
9X18 and 1/2X20

 

So I went though a similar problem with a 76. I had to replace Booster, MC, All 4 Calipers, all soft lines, hard lines in rear quarters. I kept the main lines. Since I still was having this problem where my bakes barely worked and pedal traveled to the floor super easily. I took all the rubber lines out disconnected the MC lines. I then took a air compressor and shot all the main lines out and got some crap out. Then I hooked it all up and ditched the vacuum bleeder, it was useless, and had a partner compress the brake pedal, once it’s compressed I opened the valve, let the air come out, and closed the valve, then he let off the pedal. I repeated the process until no bubbles came out. Starting on the passenger rear, than drivers rear, passenger rear, and finally the drivers front. This is for me a much more effective, fast, efficient way to go about bleeding. Plus you don’t go through near as much fluid. This was the only thing that worked for me. The vacuum bleeder didn’t work. It just sucked up endless brake fluid. The brakes work now, not good enough to skid, but I’m gonna bleed them more here soon. For now it’s good enough. Also you wanna make sure you have the right booster plunger. If it’s even off the slightest but you won’t get the immediate, and full effect of your brakes.

 

I am having the exact same issue with my 76 brakes. What did you do to get it fixed?

 

Did you fix your brake issue?