The Stovebolt Site Forums The Parking Lot Big Bolts Brakes/Hydrovac

Hi, I am new here. First post. I have a 1954 GMC 1.5 ton. I replaced all my wheel cylinders and master cylinder, ordered all new parts and rebuilt my hydrovac, adjusted shoes, and bleed the complete brake system. I drove the truck a couple times and the brakes were good. I let it sit a couple weeks and had no brakes, master cylinder was now empty. i checked all lines and bleeders for leaks and couldn't find anything. re-bleed everything again, and the same thing happened. empty master cylinder. I removed the hydrovac, and expected to find the power cylinder assembly full from an internal leak. It was empty. Can someone give me some pointers on where I am losing brake fluid? Thank you.

Check the vacuum line from the booster to the manifold for fluid.

Park it indoors and lay dry cardboard under all the tires the booster, master and any joints with Tee or other fittings. Look for stains on the cardboard.

Get someone to hold the brakes down hard with the engine running to maximize the pressure in the system and trace all the lines for leaks.

Rusty, Another thought is to check your master cylinder. If the parts were mistakenly put in wrong, or if there is seepage around the cups, there could be enough leakage to cause noticeable fluid loss. One quick way to check this would be to pull the rubber seal back from the pushrod end of the cylinder and see if there is fluid inside, indicating leakage from that end of the cylinder.