Re: Bronco Brakes
Checking the booster is easy. Run engine, shut it off in a quiet place, listen under dash for a hiss. Assuming good vacuum from engine to booster when engine is running, when you shut it off, the booster holds about 2 &1/2 brake-applications worth of vacuum. So if the booster hisses for a bit, then trails off and stops, you've got a break in the boosters diaphram.
Running the engine, then shutting it off, you should be able to press down on the brake and get a softer pedal about 2 & 1/2 times before it goes hard (to no-power assist).
A final for-sure check if you have or can borrow the equipment, is to disconnect the vacuum line from the Check Valve at the booster under the hood. connect a hand-type vacuum pump (and vac gauge if you have one) to the Check Valve. Put some vacuum on the booster. It should hold it just about forever if the diaphram is good. I troubleshot and replaced one on a full-sized 90 Merc about a year ago. The booster is easy to change, and if that is the only problem, you don't have to disconnect any brake lines. However, in the case that I worked on, the booster went at the output end, where the rod comes out to depress the piston in the Master Cylinder. And the Master had been leaking back there a while. I think the brake fluid eventually softened up the rubber, and it failed. So I put a rebuilt Master in. Toughest problem was getting the #@!**% plastic reservoir off the old one without destroying it! Yeah, the two-big-screwdrivers-and-pry method didn't work!
I think the rebuilt booster was something like $60 at AutoZone.
Have fun, and brake early! UncleDon