A buddy has changed his 61 to a dual line master cylinder. Put on a new booster and a new master cylinder. He has about 1/8 push in from the brake pedal pushrod into the booster. The output rod into the master cylinder has about 1/8 clearance in the master before it pushes the piston. The master is for a drum/drum system with no distribution valve. His brakes take a lot of pressure to stop the car. Question is, does he need to change to a smaller bore master? Or what?

 

Since you didn't tell us the bore of the old and new M/Cs, it's a little difficult to provide useful info.

First, fix the pushrod problem. Are you sure there's clearance? Typically the dual circuit M/Cs require a SHORTER pushrod than the single circuit M/Cs. This is critical, as an improper pushrod length can prevent the M/C pistons from uncovering the compensating port. If the pushrod really is too short, this is also a problem.

Second, the distribution block serves only to provide a place for the differential pressure valve that illuminates the BRAKE light on the dash as your foot goes to the floor. You don't need it and it's presence or absence has no affect on pedal pressure. It is NOT a proportioning valve.

Third, while you didn't explicitly say so, I ASSUME we're talking about a four wheel drum vehicle. If so, I ASSUME the new M/C is a drum/drum M/C with residual pressure valves in the outlet ports.

Finally, the new M/C bore should be the same as the old one. If it is, and the system is bled properly, and the pushrod is adjusted properly, there should be no difference in feel or performance.

 

That's just it, the pushrod is adjusted for proper length and clearances. and it is a drum/drum system. New master cylinder from MPP that they recommended be used. I don't know if it has residual pressure ports or not. I did not look for them when I bench bled the master. Same master bore size as the old one. Beats me what is wrong.