Interesting thread as I have the same problem now too. Have a 67 Satellite that I changed to front power disc from standard drum drum. Bought a 7 inch MBM from Nelcohotrods complete with MC (1 1/8 bore) and proportioning valve. Was out of warranty before I got to test the brakes and wow, worse than standard disc, literally could not stop this car in any reasonable distance.

Tried to get help for Nelcohotrods and they would not give any suggestions or assistance at all. Was to be made in America parts but I find out today the proportioning valve is China made.

Spoke to National Moparts today and he says those proportioning valves are mostly bad right from the start. He has replaced something like 40 brand new ones that never worked. He gave me a replacement to try and said I need to get down to a 15/16 bore too.

This supports what most of you are saying on here. Smaller bore master and check the PV.