Proportional relationships - Cars & Trucks

A Proportioning valve is required on vehicles that have disc brakes on the front wheels and drum brakes on the rear wheels. Disc brake pads are normally in contact with the disc, while the drum brake shoes are normally not in contact with the drum. ... The proportioning valve reduces the pressure to the rear brakes while keeping more pressure on the front brakes. Thus, the front brakes do more stopping than the rear.

it is a load proportioning valve that adjusts the proportion of brake pressure according to the load on the vehicle
best to get a new unit and replace it then bleed the air out of the brakes

rearline may be blocked

Usually if the car has a valve it is mounted on the firewall. But your master cylinder has 4 connections, one for each wheel. So I doubt your car has a separate proportioning valve. You could check with the dealer to verify.

H Colin, The brake proportioning valve adjust the amount of effort required for maximum efficiency with different loads in the vehicle. The brakes in most vehicles are proportioned for approximately 60% front brakes and forty percent rear. The valve makes small adjustments to braking between front and rear. Regards john

now the question is
did you get the original master cylinder as a replacement or did you get a new master cylinder for the car that the callipers came off
understanding that there is a direct relationship between master cylinder piston size and calliper piston size, you problem appears to be that there is not enough surface area in the master cylinder to effectively push the calliper pistons out to apply the correct braking pressure
mixing and matching with brakes requires a lot of hydraulic knowledge and pressure differences which include fitting a proportioning valve in the rear brake lines to prevent brake lock up at the rear wheels

All bleeding is done at each caliper and wheel cylinder. The proportioning valve has no bleeder. The proportioning valve makes the front brakes engage before the rears do to prevent the rear brakes from causing a sideways skid.

The proportioning valve varies the pressure to each wheel, usually the back wheels, because more of your braking is done with the front wheels, than the rear wheels. So the proportioning valve restricts the pressure from the master cylinder, to the rear wheels.

Proportioning valve for the brakes? rear brakes? rear brakes are located on the rear end, for all brakes(front to rear proportion) is usually part of or under the master cylinder, and if either are bad then you need a new one, there is no resetting them

The proportioning valve can be mounted on the frame or just off of the master cylinder. Yes, you can buy aftermarket and no you don't have to stay OEM on it.