wjalex4

Brake Woes. 1976 C3, manual 4-speed, power brakes. Owned it since October 2018. Brakes had been fine, but recently had pedal going to the floor.

Examined all four brakes... found one front caliper looked a little damp. I replaced both front calipers, front pads, front rotors, and did inner and outer wheel bearings and inner seals while I was at it. After bleeding, brake pedal would still sink to floor sitting with my foot on the brake, engine not running.

I removed master cylinder and found a little brake fluid where the metal rod from the booster enters the master cylinder. So, I replaced the master cylinder. Bench bled the new master cylinder until no bubbles, then installed it. Bled all four brakes with a helper in the car (pump, pump, pump, pump, hold method.) Pedal was nice and firm (engine not running.)

Then, I start the car and the brake pedal immediately sinks to the floor. Argh! Could I still have air in the lines, and I'd get a good pedal until I start the car (and the brake booster kicks in)?



I've read that C3s are a PITA to bleed --- thank you!



---Walter

Peterbuilt

Does the power booster hold vacuum?

wjalex4

Yes, the power brake booster holds vacuum, for when the engine it shut off, several minutes later I can pull the hose from the power brake booster and here it lose the suction. I'm pretty sure I have air in the lines somewhere, for I disconnected my brake lines from the master cylinder, then plugged the master cylinder outputs with two brass plugs. I then had a good hard brake pedal even with the engine started. I am going to try bleeding again with a suction bleeder at each wheel, one at a time.

Thanks!

Kim72

Yup. You still have air in the system. Lightly tap the calipers and bleed some more. Others have had good success with gravity bleeding. The job is a PITA to say the least. Keep going you will get it.

amvenne

Following because I am seeing the same issue in my '77. I found a small leak where the master lines thread into the master cylinder. Not sure which threads were messed up so I have new lines and a new master cylinder coming in. I'll let you know if that helps solve my issue.

cagotzmann

Here is the procedure I would use to bleed a system that has been opened.

1. Bleed the MC while on the car.
https://www.corvetteforum.com/forums...-cylinder.html
2. Push the caliper pistons in as far as they can go by pushing the pads and shimming between the rotor and the brake pads to hold the caliper piston in during bleeding.
3. Apply 20-25 PSI at the master cylinder to pressure bleed each caliper.

wjalex4

Thanks for the tips. I will bleed the master cylinder per the video you included, and I have a Speedibleed ordered, which will give me the same kind of the pressure bleed as you have pictured. So far I've done pump-pump-hold with a helper, gravity bled and suction bled, but still have the pedal sinks to the floor as soon as I start the car.

MelWff

you dont mention the sequence of the brake bleeding, are you starting with right rear outer, right rear inner, left rear outer, left rear inner, right front, left front?
Is the car level when you do the bleeding?

wjalex4

Great questions.

I've been bleeding in this order:

- Right rear inner, right rear outer, then repeat right rear inner, right rear outer.
- Left rear inner, left rear outer, then repeat left rear inner, left rear outer.
- Right front.
- Left front.

As far as level, I've been jacking up the rear and putting it on jack stands to bleed the rears, then put the wheels back on the rear and put it back on the ground. Then I jack up the front, put jackstands, then bleed the front. Maybe that's an issue... perhaps I need to remove the rear wheels and then lower the car as close to level as possible before bleeding? I can actually do the fronts without removing the wheels but I was giving myself more visibility since I'm having so much trouble.

I just saw a video this afternoon on re-bleeding the master cylinder on the car. The guy recommends jacking the rear up pretty high to force any master cylinder air bubbles to be up against the piston. Then the helper sits in the car and slowly depresses the brake pedal over and over again until no more tiny bubbles. I don't know if I have any air in the master cylinder or not, but if I install brass plugs into the master cylinder brake line ports I get and maintain a hard pedal even when I start the engine.

Thanks! --- walter

wjalex4

I bled the master cylinder on the car (I had previously bench bled it) last night, but the pedal still spongy. My Speedibleed should be here next week and I will start fresh bleeding the brakes. Thanks everyone for your suggestions.
---Walter

cagotzmann

Did you get any bubbles via the bleed holes in the MC ?.

wjalex4

Yes, I got some very small bubbles from the bleed holes in the master cylinder. I had the rear of the car jacked up high enough that the master cylinder was higher towards the firewall (per the video.) I then got most of the bubbles from the rear (closest to the firewall) chamber of the master cylinder.... very few from the front chamber. I'm now starting to get punch-drunk from jacking up the car, removing and reinstalling wheels, and bleeding wheel cylinders. In my 35 years of working on cars, I have never had so much trouble with a brake bleed. :-/ Thanks!

cagotzmann

At least you can know see that bench bleeding a MC isn't enough to complete the job.

When bleeding the calipers make sure you press in the pads to force the piston into the caliper as far as they can go. Hold them in place with vise grips / shim when bleeding the caliper. The most common place air is trapped is in the piston bore when starting from a open system. During this process you don't need to worry about the level at the MC. Just use pressure at the MC to force fluid out. This way when you close the bleeders they don't suck air back in as pressure is always applied to the caliper. Once the bleeder is closed then you can release the vise grips / shim of the pads to allow the MC pressure to push the piston / pads against the rotors. Also with 20-25 PSI at the MC you should also have a difficult time rotating the front rotor. Another good test to see how things are going.

Its worth making a pressure plate for the MC as it makes everything much easier. This is what a motive power bleeder does.

I found it easier to use my air compressor with the MC plate like this.

70s Vette Guy

Your issue sounds so familiar. I just spent a month trying to bleed my 73 on the weekends. Never had a problem bleeding before and I was stumped on this one. Ultimately I purchased Russell Speed Bleeders to prevent any air going back into the system when opening and closing the bleeders. With the bleeders installed I found one wheel continued to expel tiny air bubbles. What I found was one of my caliper pistons was allowing air in when the bake was released. The caliper never showed any signs of leaking fluid and was bone dry. I replaced the caliper and problem went away. This was my first time using Russell Speed Bleeders, but I am a fan. I could bleed the system without any assistance and it did make it very easy. I also used a clear piece of tubing, bought from Home Depot, and attached it to the bleeder and ran it back to the master cylinder. This allowed the fluid to make a round trip, I didn't have to worried about the master running out of fluid during the process and I didn't waste any fluid. The best thing was I could see which wheel had air moving through the line, it would also show any discoloration of fluid and debris.

I feel your pain and wish you the best.

Jim

Street Rat

Pain in the butt is what I call it.

7T1vette

Does your "BRAKE" warning light work (comes on when E-brake is applied; also comes on with brake system problems)? If that light works and it is not ON presently, the shuttle valve in the brake fluid distribution block should not be a problem. If the lamp is ON, you have a leak in the system somewhere. You've replaced MOST of the brake system. Have you replaced the rubber jumper hoses (from metal line to calipers)? If not, that could easily be your problem. Over the years, those jumper hoses break down on the inside. The hose can get weak and simply bulge when pressure is applied so that the pedal will go to the floor.

Another possibility is that the metal rod between power brake unit and master cylinder is the wrong length. Every time the M/C or the P/B unit is changed, that rod must be checked for proper length for YOUR set. It needs to be exactly the same length as the space available to fill it (+.000/-.010). I don't think that is your problem, as you had this condition before you changed parts. But, it is still a probability.

cagotzmann

Yes do this also. This is a good tip to follow.

Peterbuilt

Time for a new booster.

amvenne

As I mentioned above, I was having a similar issue. This weekend I installed a new booster and MC, and MC lines. My brakes are better, but only slightly. Now when I start the car the brake light comes on and the only stopping power comes at the very end of the brake pedal; when pressed all the way in. If I pump the brakes a few times, the pedal gets stiff and the light goes out. It feels like I have brakes. I can even hold the brake pedal in for up to at least two minutes and it doesn't lose pressure. Once I release the brakes it goes back to the same issue, pedal goes to the floor before stopping the car and the light comes back on. I cant see any brake fluid visibly leaking anywhere in the lines or at the calipers. My amature inclanation is a vacuum issue, but I checked the vacuum hose in to the booster and it has suction. Anyone else have any ideas?

cagotzmann

You have air in the MC and in the calipers. Red light = low pressure. Air compresses , brake fluid doesn't compress easily.

Brake fluids must maintain a low level of compressibility, even with varying temperatures to accommodate different environmental conditions. This is important to ensure consistent brake pedal feel. As compressibility increases, more brake pedal travel is necessary for the same amount of brake caliper piston force.