Comeaud,
I changed my master cylinder because it was leaking onto the firewall. If your master cylinder is not leaking, and even if it is, this is probably not the cause of weak braking. In the system you describe, with boosters located elsewhere, the master cylinder only needs to activate the brake boosters that do the real work of stopping the truck, so weak stopping performance is probably caused by problems in the boosters or downstream trouble with brake lines or wheel cylinders? As usual, others know more about this than I do, so I hope this will be corrected if I am wrong about it.
