OT Late 70s Chevy C50 Hydra-Vac Brakes

Tracy Brown · Sep 3, 2017

I'm helping a friend on his truck. Pedal goes to the floor first couple of times, them you have hard pedal but no brakes. Done so for, Rebuilt hydra-vac unit, replaced all rear wheel cylinders and shoes (rocks inside the hubs messed things up). Bled the brakes several times. Considering replacing both master cylinders with new and hudra-vac with new. Or, omit lower master cylinder and hydra-vac, go with power brake setup under the hood like 70s and 80s full size cars. If not enough fluid flow with the power brake set, disconnect front brakes and have rear brakes only. No need for the correct or safety police to jump in, this truck will be occasional farm use only and we jump need to stop it. Thanks for your input

farmer656 · Sep 3, 2017

adjust the brakes shoes on rear and front, this should cure your problem

Dune Country · Sep 3, 2017

some of those had 2 brake shoe adjustments, IIRC

Phil9N3667 · Sep 3, 2017

As farmer66 says, adjust the brakes, but do it one wheel at a time to the point where they'll just barely turn freely by hand. Then bleed it good, starting at the farthest from the master cylinder and working your way towards the master cylinder. If you can borrow a pressure bleeder, all the better!!
Good luck.

Welding man · Sep 3, 2017

I have worked on dozens and dozens of these trucks years ago,but we can't even get parts for them in my area. Finding the right booster is impossible, a master cylinder kit unavailable and most of those trucks have been cut up for scrap. But back when I was working on them on a regular basis. We did not have much luck bleeding them without a power bleeder. The combined volumes of the wheel cylinders and and the amount of fluid you can pump with one pump of the master cylinder makes it tough to get all the air out.Bleed the booster first, then the rears and then the fronts. Here's a quick test to try. Shut the engine off. Pump the brakes until all vacuum is gone. Pump the brakes until you have as close as you can get to a full pedal.Hold that and start the engine. You should feel the pedal drop when the vacuum comes on.If it doesn't you have a bad booster or no vacuum.You will also have this problem if the drums are badly worn. Even if the shoes are adjusted up, the pistons are so far out in the wheel cylinders you don't have enough volume of fluid to compress them all in one pump. Hopes this helps.

Hobo,NC · Sep 3, 2017

Pedal goes to the floor first couple of times, them you have hard pedal but no brakes.

That's the way it is if the brakes are out of adjustment are you have air trapped somewhere in the system... Its the nature of the beast it will run out of assist... You may get 2 pumps and possibly 3 after that hold on....

I know 60's and some 70's had dual wheel cylinder front and back. The rears were no problem to bleed but the front were. Its one of the rare ones I used a pump up sprayer and back blend the front brakes.

You can soft pinch the rubber brake lines to eliminate issues up stream of the wheels. If you now have a firm pedal release one line at a time till you find the wheel that has the issue. are make some block off plugs, plug the brake lines off going to the wheels at the Booster to eliminate from the booster to the master.

ohiojim · Sep 3, 2017

good luck, I drove those chevy's back in the day and brakes were never what they should be. mechanic said the only way to bleed was with a power bleeder

Bill in Colo · Sep 3, 2017

the only way I've had any luck is with the power bleed.. I have a 1972 GMC C50 with 31,000 miles the truck is used to haul water every two months for a total of 80 miles. I replace vacuum boster 3 years ago.

Steve@Advance · Sep 3, 2017

Sounds like air in the lower system, or brakes too loose.

When you pump the upper cylinder a few times and get hard pedal, but no brakes, the lower cylinder is bottomed out and still no pressure to the wheel cylinders.

Verify the brakes are adjusted, then try power bleeding. Might try raising the front of the truck, power bleed from each rear bleeder up to the HV. Then raise the back, power bleed the fronts. A power bleeder can be a new (oil free) pump type oil can, or a pump sprayer. Just be sure not to introduce any more air.

2x4 · Sep 3, 2017

isn't there a bleeder screw on top of the hydro-vac? Overlooked sometimes.

K Effective · Sep 3, 2017

The procedure would be to bleed the hydro first, then the farthest wheel next, etc. A lot of times the air gets into the hydro line during bleeding of the wheels when the master cylinder is drained.

OT Late 70s Chevy C50 Hydra-Vac Brakes

Tracy Brown

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farmer656

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Dune Country

Well-known Member

Phil9N3667

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Welding man

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Hobo,NC

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ohiojim

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Bill in Colo

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Steve@Advance

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2x4

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K Effective

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