Brake Problem

[kevance]

Need some help on brakes. I have front disc and back drum set up on my 64 F100. The problem is I have a hard brake pedal and while the vehicle stops it doesn't stop on a dime. There are no fluid leaks and the check valve on the booster is fine. I have checked the booster by pumping the brakes prior to starting the motor, maintaining pressure on the pedal and then starting the motor and the pedal drops about 1/2 an inch at best. I have yet to take the truck on the road and have only had it in the driveway. The way it stops in the driveway going 3 mph, I figure I need a football field to stop if I'm going 30 mph. Could the booster be undersized? It was bought specifically for the vehicle but that doesn't really mean anything I guess. Could I not be pulling enough vacuum off the motor? (bored out 302 with a camper cam and I don't have a gauge to see if I'm pulling 18") Is there an auxiliary vacuum pump that I could incorporate to the system I already have if I need more vacuum? Any suggestions would be greatly appreciated as this has me boggled. Thanks in advance for the advice or suggestion.

[DV65CustomCab]

What calipers and master cylinder are you using? It sounds like maybe the master cyl bore size isn't correct.

[kevance]

I am not sure of the master cylinder size but the calibers were part of a mustang II front end kit I put on the truck so I would hope they are the correct size. How can I tell what the bore size is on the master cylinder?

[rbeck204]

You might also check the PSI rating of the new booster.

I had my stock 1961 Thunderbird brake booster rebuilt since I was converting to front disc brake and dual master cylinder. After all the new stuff was put back on the Bird, the brakes felt like manual brakes. I checked and rechecked everything. I finally went back to the Brake Booster rebuilder (a company in business in Sacramento for over 50 years) and he immediately knew the answer. The booster was originally designed for 400 PSI and that was exactly what I was getting. The booster would have worked perfect if I had left the drums on the front. The problem was that the discs required a booster that would put out 800PSI. I changed the booster to one designed for discs and the car stops on a dime with very little pedal pressure.

Just a thought.

Rich

[kevance]

The bore size appears to be 1 1/8 on the master cylinder. I think my booster is undersized like the one guy replied as everything seems to working properly but the system doesn't seem to have the muscle. Do they make any kind od electrical assist to what I already have installed? or does it all depend on the booster size anyway?

[unibody madness]

Length of your actuator rods, both pedal to vac assist, and vac assist to master, is critical and can cause that very problem.
I set both of mine to minimum clearance, and solved a similar issue.
John