the one in the back is not a proportioning valve but a pressure valve. it keeps the fluid from leaking back into the master cylinder. if you remove it your brake peddle will feel squishy and you will lock up your front brakes all the time. you can get a inline replacement from summit that are a set pressure. I've used them some rods and customs I have built. or you can get an ajustable one from Willwood that is a proportioning valve