The Stovebolt Site Forums The Shop Area Driveline Power brakes lacking power after one push

1958 Chevrolet 3200. Upgraded to power drum brakes and dual master from I think CPP. (Been a while since upgrade)

Problem: Driving along and hitting the brakes they grab well with power, but if held or reapplied right away I have to stand on the brakes. It lacks all power after one press. Any ideas? The brakes are very touchy when just leaving my drive way, but after going down the road a ways or after the first press of the peddle it lacks power.

Engine has good vacuum at about 21.5" at idle. Vacuum line runs from stock intake manifold, T's off for wipers, and then runs to the booster. Booster holds vacuum after engine shut off. Can't blow air into booster, so check valve must be working. After shutting off engine, brakes feel to have power on first press, less and less with subsequent applications.

Stock setups usually have a BIG tube going to the vacuum booster. If your line is too small, it may not be able to recharge the booster quick enough due to friction losses in the small line.

How big of a line is needed? I have been using the line that came with the booster. But, it was necked down at the manifold to the original wiper hook up.

I just completed a trip to Home Depot and got some brass fittings. I now have 3/8"s coming out of the manifold and supplying the booster, with a T that necks down to a 1/4 for the wipers.

3/8" from manifold to booster is minimum.

What engine are you running? If it's a six, you might need a reservoir tank to retain enough vacuum, especially if you're using one of the tiny boosters with a 6 or 7 inch diaphragm. The big vacuum line will help, but I'd suggest putting a piece pf PVC pipe a couple of feet long with caps glued on both ends under the cab inside a frame rail, and routing the vacuum from the engine to it through a check valve, and plumbing a line from the reservoir to the booster. You'll get 2 or maybe 3 brake applications before you run low on vacuum. Paint the pipe flat black and it will virtually disappear.
Jerry

I think the line is 3/8" now, and with the new "T" I made it might supply the required volumn!



I am running a 235 bored .40 over, with the Corvette 235 cam you have suggested before. I was thinking of a reservoir tank myself, but not making my own. I am using the smaller diaphragm booster, so I will try a test run with the new "T" off the manifold and if it doesn't work I will try making a reservoir tank.


Thank you all for the ideas!

I just installed an 8" dual diaphragm booster with a dual MC on a 58 GMC 100 with drum/drum. I wouldn't recommend one of the smaller single diaphragm boosters.
Fred

I added a reservoir tank (using an old MAP cylinder) on my Binder, (327 SBC) and it worked very well.

So, like an idiot, I didn't check all my 4 drum brakes before posting. I found the driver's side rear drum had a slight leak. Not enough to dribble onto the ground. Found it when I was taking all 4 corners apart and found alittle liquid on the E-brake bar. The Brake Cylinder was leaking. Not surprising since the truck has sat for about six years. I replaced both rear brake cylinders and took it for a drive. Holy Cow! I was locking them up with out issue! Now I noticed the boot on the brake booster has broken and needs replacement. It''s a 7-8" brake booster from POL online. Anyone know where I can get one? One from a 70's chevy looks like it would work?