bob 71 Posted March 21, 2021 Share Posted March 21, 2021 i had problems since day 1 383 stroker 487i 503e low vacuum alot of over lap with cam came with vac pump and canister drove it for a couple years untill it got worse put in stock replacement power booster seemed ok/like i was used to master cylinder failed pedal to floor put in new rebuilt master brakes never felt right put in new cpp master/booster/portioning valve new vac pump still running same vac canister brake pedal still hard sometimes i know SOMETIMES drove car around neighbor hood today tried to power brake/launch/burnout brakes wouldnt hold twice held ok 3min later in driveway bad canister maybe? Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted March 21, 2021 Share Posted March 21, 2021 Do you still have the vacuum pump you mentioned? A vacuum can usually is good for 2-3 pumps of the pedal before you get a hard pedal. How are the rubber brake hoses? With age and today's brake fluid the rubber hoses tend to close up internally. I have a comp cams vacuum pump on my 70 SS with a 496 and dual diaphragm 9" booster. No hard brake pedal ever. Some noise with the pump running but the 496 drowns it out. Lolol 2 Quote Link to comment Share on other sites More sharing options...
bob 71 Posted March 21, 2021 Author Share Posted March 21, 2021 still have pump and canister has good vacuum 15 inches. booster not connected to engine no idea what brand canister. car was restored/redone before i bought it. had crate engine rebuilt t350 paint suspension brakes including hoses not exactly sure how many miles on rebuild. ive put about 25000 since 2015 Quote Link to comment Share on other sites More sharing options...
jft69z Posted March 21, 2021 Share Posted March 21, 2021 15 inches is typically on the low side for power brakes. I just went thru this same situation on my car, 15 inches at idle also, to be exact. I tried a couple of different power brake boosters, both after market and a genuine GM. Still same, hard pedal. I got a Leeds vacuum pump that puts out around 21 inches of vacuum and that made a world of difference. Chances are, even though you have a vacuum pump, it may be going weak from the sound of it if you're only getting 15" (or a leak somewhere). I tee'd mine in with engine vacuum (with extra check valves to stop reverse flow) and that helped take some of the constant load off the electric pump. Part throttle cruise typically produces good vacuum, so I figured why not use it. All of the brake parts vendors I worked with agreed that 15" was a little low, with 18" or more being better, btw. The other thing you can do to get a better brake pedal is to drop down on your master cylinder bore size. The Monte Carlo with power brakes comes standard with 1 1/8" bore, you could drop down to a 1" bore to help get your brake pressures up. I ended up doing that also with very good results. Quote Link to comment Share on other sites More sharing options...
bob 71 Posted March 22, 2021 Author Share Posted March 22, 2021 found a small vac leak at canister. i will try that then go from there. thanks for all the help/ideas Quote Link to comment Share on other sites More sharing options...
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