Does anyone have any idea how much vacuum the double diaphragm vacuum pumps pull?
I've been having some trouble stopping, like downshifting and both feet on the brake pedal. After replacing all the wheel cylinders twice and the master cylinder and booster once, I decided to check the vacuum pumps.

I tightened up a few screws and replaced the o-rings; it stops a whole lot better now but I still can't lock them up. That old 74 'Binder has drums all around. Should I be able to lock up the brakes or am I probably getting the best I'm going to get without disk brakes?

 

Minimum spec is 8.5", but most of them will pull close to 20".

 

My single diaphragm (one of the two = I had to lose one to make the engine fit) easily holds around 25"hg with quick recovery.

I don't yet have gauges on the other two trucks that have double-diaphragm pumps.


If checking with a hand-held gauge, check each diaphragm independently, as either one can be a dud and the other still good.


I found the operating-rod absent from the one I removed, so it was just going along for the ride.



If you have one good and one bad, you can isolate the bad one and use only the good one.

 

I figured you two guys would know.
Thanks, I'll get a gauge and check them.
I'm using my calibrated forefinger over the intake tube on each of them separately with the engine running and getting a clear pop when I pull it off.

There's a guy in Michigan that thinks he can build disc brake brackets for that Dana 70 closed knuckle front axle. I am real temptedto shell out the 5$575.

 

Wait a minute, BK. 25" of hg or H2O?

 

Vacuum in measured in inches of Mercury.

hg is the chemical symbol for Mercury.

 

Before spending too big a fortune, look into the possibility of using late-70s/early-80s 3/4-ton Chevy rotors and custom-built caliper brackets.

Nearly all of the 8-lug disc-brake conversion kits are based upon using the rotors from those year GM 4x4s.

 

Thanks, BK.
The guy in Michigan uses GM rotors and calipers and cuts the brackets from 3/8 plate. The brackets replace the backing plates. He says he can supply the whole assembly, calipers, rotors, and brackets for both wheels, for $575. Is that high? I'll have to get the hubs turned down a little.
I also thought about changing to a hydrabrake setup instead of the vacuum assist. I remember you mentioning in one of your posts that they'll throw you through the windshield.

I better track down some prices.

 

YES, they will.

I haven't yet had the opportunity to convert my truck over to Hydra-Boost; but, in my daily business, I have reason to be in and move dozens of different makes/models of vehicles every day; probably the quickest applying brakes on the road are those on the "SuperDuty" Ford diesels, with the occassional GM product running a close second; THEY WILL STOP.




Another thing you can look into is old timey "booster brakes".

What these consist of is a huge frame-mounted vacuum-booster/diaphragm that charges a seperate master-cylinder.

The same system is used on vacuum-over-hydraulic trailer-brakes and they are pretty much bullet-proof.


Many older "two-ton" trucks, with hydraulic brakes, had these units, particularly in the sixties/seventies.


These will out-perform any disc-brake set-up on the road, but they are deadly on slick roads as they work TOO GOOD.

 

I bought a vacuum gauge and checked both pumps; 20"hg on one and 19" on the other. Thanks for the info.
I'll look into that vacuum over hydraulic system you described, BearKiller. There's a salvage yard with old trucks in it not too far away.