I bought a 1967 GMC 960 truck. It is equipped with a brake vacuum canister. I have to pump the brakes 3 times before the work. No brakes at all before I pump. But after pumping 3 times brakes are good.
Sounds like you have air in the brake fluid lines that needs to be bled. What makes you suspect the vacuum canister?
Have already bled all the brakes lines. Still the same problem
reseal the master cylinder, the seals get weak and need to be exercised a little to get it to hold. Unless your pedal feels like there's no boost, like the engine is off, or that boost changes, then I wouldn't suspect the vacuum booster.
Good chance you need to adjust the brakes, if there's a lot of slack between the shoes and drums you can pump them out and the check valve in the master cylinder will hold them for a while but they eventually bleed all the way off. Just a guess. Adjusters may be rusted solid, that trucks pushing fifty!
Good point. If they need to be adjusted, or there's air, then the level will go down slightly in the reservoir after the three pumps and come back up slow. If the cup seals aren't catching the first time, then the level won't be down much at all and come back immediately after the pedal is released. Maybe use a ruler on a slant in the reservoir for an "inclined brake fluid level meter".
X3 on the adjustment.