[JDPascal]

MagicRat is right, there might be a “residual pressure check valve” in the master cylinder port that you connected to the front brakes. Normally this is in the line to the rear drum brakes but sometimes things get mixed.

As likely as this is the cause, the first check to make is for brake pedal free play. With a power brake system this is likely not a problem but check anyway. You need 1-2 inches at the pedal pad.

Second check is to take off the master cylinder cover and have someone step on the brake pedal while you watch from a safe distance. What you are looking for is a swirl (gyser or spirt) from the reservoir when the pedal is pushed so protect the paint.

If there is a swirl (gyser or spirt), then it likely is the valve above.

If no swirl (gyser or spirt), then you need to check the placement of the pushrod from the booster to the mastercylinder or a small rubber disc inside the booster where the pushrod sits.

If these are incorrect, the piston is held forward in the bore and blocks a small port in the bottom of the reservoir that allows the fluid to expand when heated without applying the brakes. Blocked, it traps the heated fluid that expands and the brakes lockup after it is driven for aways.

One other cause is a deteriorated brake hose where the inside makes a flap into a oneway check valve that does the same things as I described above – but you said you changed the hoses???

Hope this helps.

JD