Brake system hydraulics.

[ericjon262]

Specifically, the combination valve and master cylinder.

My understanding is that the combination valve is supposed to shift to isolate the front or rear brakes in the event of a blown hose, however, I thought this was also the whole idea behind modern master cylinders having tandem pistons, each piston feeding half of the brakes, front or rear. Yes most brake master cylinders have a common reservoir that can be drained, but they're typically divided midway up such that a leak on one side doesn't drain fluid from both.

The whole system seems very belt and suspenders, not that it would be a bad thing to have redundancy, but I just don't see the advantage.

is my understanding grossly flawed? the reason I ask, is because I'm planning on installing a line lock, and was considering an adjustable proportioning valve in the rear circuit, and eliminate the combination valve from the equation. Am I making a grand mistake by doing this?

[neophile_17]

Hello,

There is more involved in a well functioning braking system than most hotrodders understand. This is what I have learned trying to improve our braking system.

The combination valve does 3 things.
Proportioning
Indication
Timing

Proportioning adjusts the front/rear bias.

Indication lights the light to let you know something is wrong.

Timing adjusts how pressure is applied to the front/rear so that the car behaves well under braking. This is most evident in F disc R drum cars where it is needed to avoid excessive dive. The rear brakes are applied first until a certain pressure is reached and then the front brakes are applied.

Many GM cars use step bore master cylinder which at least doubles the complexity of designing a competent brake system. Our race car is (now) a single bore master because getting the rest of the variables right is hard enough.

If your system is similar enough to stock the combination valve is the path of least resistance. If your caliper bores and tire heights are different enough then removing the stock valve and using an adjustable proportioning valve is the second easiest. Pay attention to which port on the master cylinder is attached to the the front/rear. Many other cars are the reverse of the Fiero and where the ports are drilled affects front/rear apply timing.

Good luck!

[ericjon262]

I think I may try and build and install one of Aaron88's adjusters in the stock combo valve, I put together a bracket to hold a line lock using the master cylinder bolts, and bent lines to a stock combination valve, so ditching it now doesn't make a ton of sense.

[Series8217]

I'll put my aaron88 brake bias adjuster up for sale after I have my new brake system installed this spring, if you can wait that long.

[ericjon262]

I'll put my aaron88 brake bias adjuster up for sale after I have my new brake system installed this spring, if you can wait that long.

I may take you up on that, let me know when the time comes.