Bench testing a brake booster?

[twistedtree]

I'm wondering how practical it is to bench-test a brake booster, and am wondering what sort of fixture would be needed.  I think I have a bad booster on one of my cars (takes lots of peddle force to brake the car), and I have three boosters in my parts pile, but I don't know if any of them work.  Given the high cost of replacements, I'd like to confirm if the one on my car is really bad, and I'd love to figure out if the spares that I have are usable or just cores for rebuild.

Has anyone sorted out how to bench test a booster?   Is testing for vacuum leaks all that needs to be tested, or is there a practical boost test that can be conducted?

I'm thinking that a vacuum pump, some fittings, and a gauge could be used to test for vacuum leakage?

And perhaps a spare master cylinder with an attached hydraulic pressure gauge to test boost pressure?

I've got a peddle assembly and master cylinder kicking around, along with a hydraulic pressure gauge and plenty of assorted fittings which could be the basis for a test fixture.

Or is there a simpler way to 80%-90% test them without actually installing it in the car and trying it.  Or maybe a swap-out is the easiest way to test?

[Benz Dr.]

If you can press on the brake pedal as hard as you can and the car barely slows down, you can pretty much bet it's a brake booster. Bad calipers will still give you some braking or the car will tend to pull to one side. The brakes may smoke or get hot with caliper problems while you shouldn't see this with a bad booster.
 
If you have ever had you car stall and run out of booster vacuum you will know how it feels - the car won't stop. The best method to test a booster is right on the car. It's not that hard to remove one and try another. You should be aware that there are two different brake boosters used on 113's. Get them mixed up and you will have just the opposite problem - the brakes won't release. Replace the one you take off with the same type and you should be OK.

[lycoming]

I Have a 68 250  my issue is the brakes wont release. then you pull the vacumn hose off they release. I see you mentioned that if its the wrong booster it will not release. The brakes seem to have vacumn assist  Ive had the booster at cardone twice but to no avail.  Any suggestions  thanks.

[Bonanza]

If your booster is gone, when you apply the brakes when the car is stationary (and running) your idle will jump up about 300 RPM due to the vacume leak of the booster diaphram. Pull the booster out and you will probably find brake fluid in the booster. If this is the case you will need to replace the master cylinder as well. As the master cylinder ages the seals start to go and the vacume in the booster pulls the brake fluid thru and destroys the seals in the booster. I got my booster and master cylinder thru Millers, they were OEM and price was fractions of the MB prices!........Good Luck

[glenn]

Re: 300 rpm increase.   You've a 300 rpm rich mixture???

[Bonanza]

If you pull the vacume line off a distributor when you are timming your car you will notice that the RPM's jump. When you hit the brake pedal on a car that has vacume boost (power brakes)  you open a vacume switch in the booster that allows the diaphrame of the booster to push the main shaft of the master cylinder to give you "THE BOOST" If the brake booster is bad you have a vacume leak, and that is why the RPM's jump. More air -leaner mix -more RPM

[glenn]

That means the mixture was too rich to begin with...  Adding air - in this situation-  if rpm goes up 300, means it was rich.  If the fuel/air mix was peak, adding air would lower rpm.

[Bonanza]

Glen I posted a reply to a member who had a problem with his brake booster as I had with my 230 SL.