I need to pick some brains...

The Cleveland I stuffed into my '64 won't pull enough vacuum to run my power brakes, so I installed a SSB vacuum pump to do the job. It's running continously; If I block off the booster it pumps right up in no time flat. My question: How many inches of vacuum should the booster be able to hold without leaking?

Some background on the setup in case anyone has any suggestions... The brakes setup is a Master Power Brakes front power-disc conversion for the '64 solid front axle. They shipped it with too short of a Master Cylinder pushrod, so I have lenthened it a tad, and I made sure the pedal under the dash didn't have any preload on the booster. The truck stops fine. I have a Tee in the line to run a heater-core shutoff vacuum motor, and I installed a vacuum gauge in the line to run some tests. With no help the pump can hold 15" of vacuum in the system. If I clamp off the hose between the tee and the booster it pulls 20" in about 1 second and shuts off (not much volume in the hose that way!) if clamp it on the pump side of the tee and vacuum sensor it takes about 2 seconds to leak down the 15" of vacuum, so I'm reasonably sure the problem is the booster itself.

I have exactly zero experience with a power brakes and vacuum pumps; can anyone tell me if there is a problem with the booster or if this sounds normal? I really didn't expect the pump to be running all the time...

Thanks in advance...

(You can see the setup in the picture)

 

Nice setup, John.

I agree, the pump shouldn't run constantly. The vacuum shouldn't leak down in two seconds, either. I'll bet you have a booster leak. Try sticking your head under the dash while the engine is idleing, and listen for a hissing noise. Then try pushing on the brake pedal with your hand, and listen for a leak. If you hear a leak, try replacing the booster. A good booster should be able to hold vacuum for a considerable length of time.

It could also be a bad booster check valve. I would check it first. Also make sure the heater control valve isn't leaking.

 

Thanks for the reply!

The heater control valve is fine - it holds vacuume like a champ when I block off the booster. Unfortunetly the booster has a 4" stand-off - it doesn't bolt directly to the firewall. So when I'm trying to listen for leaks my ear is right next to the pump (which isn't all that quiet BTW, in case someone was thinking about getting one!). Since the pump is running against it continously and the rest of the system holds pressure I wouldn't think the check valve would really make that big a difference would it? I'm thinking you are correct about the booster leaking - I need to dig out my paperwork on the MPB kit and see if it's still covered...

 

John, what you say makes sense about the check valve; a faulty one would only cause vacuum loss when the engine was under load. I'd take a look at it anyway, just to rule it out.

When it was newer, the booster on my Bronco would hold vacuum overnight.

Do you have an aftermarket cam in the Cleveland? I'm wondering why it won't pull enough vacuum for the brakes......? Maybe check the pushrod from the brake pedal to the booster........? I'm thinking maybe it's slightly pushing against the booster and relieving vacuum somehow, yet not applying master cylinder pressure.

 

I thought of the pre-load thing, so I unbolted the pushrod from the pedal when I took the pressure measurements - there was no pre-load. I also put some silicone around where the booster and master cyl. bolt together, just for grins - it didn't help.

The Cleveland has a Comp Cams 280H cam... .530 lift, 280 advertised duration, and 110 lobe centers; I didn't expect there to be much vacuum when I fired it up!