Getting ready to replace the front disk brakes on my 75 FJ40, and wanted to see if I should also be upgrading my mater cylinder for the best performance?
My experience has been that the rear wheels will lock way too early without a proportioning valve in the rear circuit once you have front disc brakes.when I did the Man-a-Fre front disk conversion on my 40, the only thing that was required of the brake master cylinder was removal of the residual valve thingie behind the big hex nut outlet for the front brake circuit - and proper bleeding the air out of the entire system
if the rear still retains the drums, the rear circuit at the master is left untouched - once the rear gets converted to disk brakes, that residual valve needs to be taken out as well, and an external proportioning valve is typically needed to dial in the relative braking force towards both axles
My experience has been that the rear wheels will lock way too early without a proportioning valve in the rear circuit once you have front disc brakes.
X2 on residual valve removal… although they are now much more powerful with an 80 series master & booster. That said, I never had any complaints about a 40 master until the booster went out.