Hi- I own a 1973 Ford/Grumman Wonder Bread van, converted to a motorhome. The rig has a Cummins 5.9 turbodiesel installed by the P.O., with a 5-spd, & 2-sp rear end. 21.5" wheels, dually. It's mostly aluminum, the front "clip" is steel with a fiberglass engine cowl. I have a vacuum leak somewhere in the vacuum assisted brake system. It has a quick loss of vacuum boost when the brake pedal is depressed. There are no vacuum lines at the master cylinder or inside the firewall at the pedal linkage, rather there is a tank (or two) near the rear axle which seem to be brake vacuum accumulator tanks.

PROBLEM: I lose all vac. assist after one good braking action and can see the vac. gauge drop to zero in 1-2 seconds while my foot is on the pedal, regardless of how hard or far I depress it. AND since I am not 18 anymore and can't push hard enough to stop quickly without assist more than once or twice, so maybe the problem is in my leg!

QUESTION: How does the vacuum assist get triggered to go to work helping apply brake pressure? Where is the most likely place for this circuit to leak vac. pressure? AND how to troubleshot? BIG Questions, I know but there's somebody out there who has the answers! Anyone know this brake system that can help on this oddity? Thanks!

 

You must have a frame mount vaccum booster which is activated by brake line pressure.most likely you problem

 

I second what Ray said. The unit is called a hydro-vac.Its your power brakes. What you describe is very common when they fail. A very good bit of advice, do not drive this vehicle until you get it fixed! The boosters not going to work, and you can push that pedal as hard as you can, its not gonna stop. Note you should be able to get a rebuilt hydro-vac at a truck parts house or napa.

 

The vehicle is registerd by its 11 digit VIN

If the VIN begins with P4 = P400.

1969/76 P400's with P/B came with a Bendix model 2504635 dash mounted P/B booster.
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If the VIN begins with P5 = P500.

1969/76 P500's with P/B came with a Midland-Ross model C-462-C frame mounted Hydro-Vac P/B booster.

TEAA2005B .. P/B Booster ~ Hydro-Vac / Obsolete

No Ford Dealer or obsolete parts vendor has any.
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B3T2309B .. Hydro-Vac Master Repair Kit / Obsolete

GREEN SALES CO. in Cincinnati OH has TWO = 800-543-4959.

Hydro-Vac's are getting hard to find. There used to be one shop in downtown LA that had Hydro-Vac's. They are now out of business.

The last Hydro-Vac I needed, I found in IL

 

We used to get reman brake valves from a place called bepco. They were I beleive in the carolinas. They used to have a lot of stuff. Worth a shot if you cant find one. Good luck.