Power brake booster

[40cpe]

The current thread on power brake boost brings my issue to mind. I've started another thread so as not to confuse the two symptoms.

The booster on my '55 with a stock 272 works wonderfully most of the time. Occasionally when coming to a slow stop I will release the brake partially and then reapply. Sometimes on re-application the boost is gone. I'm moving slowly at these times and am able to increase the pressure on the pedal to stop. Single and double diaphragm, 7 and 8 inch boosters are mentioned. Can the booster be identified by these pictures? The best I can tell it is 7" at the larger section before the step-down. This seems more prevalent when the AC is on and the engine idle is lower. I've been thinking of a reserve canister and a convenient space is nearby in the fender well. Any other options for me?

 

[nascardave]

Hi, That looks like a 7" dual diaphragm booster. It sounds like you have a vacuum issue I would start by checking tune up, timing, valve adjustment, carburetor adjustment, vacuum source should be off the manifold if possible. You would like to have 16 to 19" of vacuum at idle, in gear if it is an automatic. Also check the hose. Sometimes people use fuel hose instead of vacuum hose. Fuel hose collapses after a while. Dave

 

[40cpe]

Thanks, Dave, for the suggestions. I bought the car this way, supposed to be a 272" Y-Block professionally rebuilt engine although I don't know the rebuilder's credentials. Reportedly it has about 4000 miles on it. New rebuilt distributor ('59 model). New wires/plugs. New 3/8" hose connected to a fitting on the intake, doesn't state fuel or vacuum on it. Carb is adjusted to smoothest idle. I put the vacuum gauge on it and it was flickering at 12-13" with timing at 10 degrees by the timing marks. I bumped timing to vacuum about 17" and back to 16" steady and that set the timing at about 22 degrees. It misses at about 3000 sitting still now, maybe fighting itself. I'll have to put it on the road to see how it does under load. Checking the valve settings is next. Now I'm concerned about correct installation of the timing chain. Thank you for your insight. Gene

 

[dmsfrr]

Adjusting the timing...
Does it have points in the distributor or a conversion kit?
If it's got points the gap/dwell needs to be correct first, or you're not on good ground to start with.

Timing chain...
Don't think about this unless/until you have to.
.

 

[40cpe]

It is points, dwell is 28 by my meter.