in response to a message on the NCRS # 12042
.? 63 best way to bench test the Power Brake Booster

Monday, 5 March 2001

Sir: I rebuilt 63 and 64-67 PB boosters for many years, until my insurance carrier declined to provide liability coverage. That is when I retired. You will need a simple “Mighty Vac”TM vacuum pump. It must be the type with an indicator dial to measure the vacuum.

Hook the “Mighty Vac” to the hose attached to the check valve. Pump it “out” until the needle reads about midnight and wait. If the needle moves, it is leaking down too soon. Could be two possibilities: 1) main diaphragm is shot or 2) front cup inner or outer seals are shot. On 64-67 only one seal used in front.

If the needle does not move, it is holding vacuum. Next, push the rod at the rear where the clevis attaches. Once you overcome the resistance from the main diaphragm return spring, the vacuum assist will allow that rod to continue forward and then it reverse and will return to the resting position. This will be accompanied by a hissing sound as the vacuum is lost. At this point without vacuum assist, it is much harder to push the rear rod. All other check can only be accomplished by disassembly. No kidding, at one time I could assemble these blindfolded and still have lots of spares – but now Nationwide is on my side – so I am retired.

I will alert you to one potential problem that I found by accident and never saw in any service manual anywhere. Pay particular attention to the circular retaining clip that holds the piston assembly in the rear of a 63 or 64 master cylinder. (Any master cylinder with a 7/8 bore.) If you have PB, do not use a retaining clip with the small end tabs containing round holes. These holes facilitate removal with those special locking ring pliers, but the tabs interfere with and rub against the PB plastic piston support. If these clips are used, they will produce a groove in the plastic that supports the front PB push rod. Every time you step on the brakes, the PB piston pushes into the rear of the master cylinder and scratches against the 2 tabs these holes are located in. Eventually the groove becomes deep enough and the front seal will not hold vacuum and pedal effort is increased. Additionally it sucks brake fluid out of the master cylinder into the front chamber of the PB booster. Messy and dangerous