Question about brake system

[raggedjim]

Do any of you guys know how a 2 circuit brake system works? A guy was asking me and I thought that if a line busted on the front or rear the other would still work. Sounded good until he asked me "Then why when you bleed your brakes does the petal go to the floor when you crack the bleeder?"

I'm not as smart about brakes as I thought...

Let me know, Rg

[dwcsr]

If he means dual system as in what a car has. In the proportioning valve is a sliding valve that when no pressure in one side of the system it shifts and turns off flow to that side. That’s the nut shell version.

Heres the long version

The master cylinder is a hydraulic pump. It contains 2 pistons both have a spring in front of them. The springs regulate fluid flow, how much and when in the stroke it flows. The pistons push fluid.

When you step on the pedal the existing fluid that is between the 2 pistons and between the front piston and housing end flows into the lines, simple enough. The springs play a roll in regulating how much and at what pressure it flows to either the front or back lines. They also play a role in getting the fluid back out of the lines when you let off the brakes.

Take a master that has 1" bore and a heavier spring in the front than in the rear, the front spring has a 10 lb resistance and the rear spring has a 8lb resistance. The rear brakes are connected to the front of the master and the front brakes are connected to the rear of the master. This is only an example.

By stepping on the pedal you are raising the master cylinder pressure in the rear chamber forcing the existing fluid against the front piston and at the same time collapsing the lighter rear spring. What this does is allows the front brakes to start applying first, once sufficient pressure has built up in the front brake circuit the front piston is forced forward by hydraulic pressure overcoming the front spring resistance there by applying the rear brakes. This action is further regulated or fine tuned by the proportioning valve or combination valve. The combination valve has the sliding valve that I mentioned earlier, it has several functions; one is to assist in the regulation of the front rear brake application and also a safety valve in case you break a line.

This more closely answers your question

With brakes applied, cylinder pressures up, and wheels are locked. Let say that you break the front left brake line, cylinder pressure in the master for the back chamber in the master goes to zero and the pedal goes almost to the floor. Now because you have no pressure between the pistons you have only the spring to assist in getting the rear brakes applied. At this point the pedal is almost to the floor, the sliding valve has shifted to close off the zero pressure side. You lift your pedal to pump the system up the springs push every thing back to start position and it draws fluid from the master for that broken side. Because the line is blocked at the valve it does not suck air, and fills the rear chamber that was just at zero pressure and you then by applying the brakes again create the resistance not from the front brake cylinders being fully applied but getting that resistance from that circuit being closed. Remember that the master doesn’t know how the other end of the circuit works it only knows that it needs a resistance of pressure to push the other piston forward. You step on the pedal again and its higher and the rear pressurized circuit pushes the front piston to apply the rear brakes.

Its intent is so you regain enough brake pedal to stop the vehicle. That’s one reason to have a prop valve in your system. This is how new parts work old crusty 1978 valve may not work at all. Some of you may remember that little tool to clip onto the prop valve when bleeding brakes so it wouldn’t shift that valve and cut off one side or the other. If you can bleed brakes with out using that clip and it does not shifting that valve it’s most likely a bad valve. it also doesn't work well if the back brakes are out of adjustment.

[smbrouss70]

Just from personal experience, I can tell you that with a VERY leaky rear wheel cylinder (similar to a busted line), I drove around for probably about 2 weeks with no fluid in the reservoir for my back brakes and the front disc brakes stopped me fine. It was actually hard to believe that I had NO back brakes because the truck still stopped fairly well.

[dwcsr]

Had it been the front leaking that badly you would have had a much different experince. Back brakes on some of these truck do very little. On my 92 I replaced the front 3 time before doing the backs once.

[smbrouss70]

Yeah, I know, but I would've noticed it sooner too

[dwcsr]

I can see the helicopter eye in the sky video now with your back end sticking out of a Subway shop. The news blip is "Subway's only Drive thru in LA. just opened! -- news at 11"

[smbrouss70]