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.

[jelly2m81]

The Proportioning / combination valve is a safety feature of times past, now displaced by an ABS module.

It still has a purpose, , install your line lock After the valve.

Lets be real, we all like to modify, like to do stupid shit in our cars, but in the end we don't want an un avoidable accident

[ericjon262]

The Proportioning / combination valve is a safety feature of times past, now displaced by an ABS module.

It still has a purpose, , install your line lock After the valve.

Lets be real, we all like to modify, like to do stupid shit in our cars, but in the end we don't want an un avoidable accident

right, but I guess I'm still having trouble understanding how exactly it performs it's safety function,

I get the whole "the shuttle shifts to block the port of the blown line" comment, but the brakes are also divided upstream of that in the master cylinder, if it were a single piston master then there would be no questions.

At the moment, I have Steven's old bias adjuster, I plan to install it, and my line lock all at once, but I have a pretty long list of things I need to make happen before I install them.

[jelly2m81]

Short and quick of this, as I only post stuff anymore when I am on the booze, so take anything I post at that value....

the MC has a front and rear circuit, the proportioning valve separates left / right front and rear.

its known that front brakes give the higher proportion of braking effect, front brake mean more that rear brakes generally.
The proportioning valve can separate the Left front and Right front brakes . ( I'm pretty sure )

[ericjon262]

I'm pretty sure that's not the case. the front two lines do in fact tee off of the combination valve, to some small degree the valve may shift and block one of the two fronts, but that doesn't seem likely based on the parts diagrams I've seen, and there is only one rear line output, that tees in the passenger rear wheel well on 84-87 (I think 88 as well). FieroObsessed made posted this cutaway on the other forum,

https://www.fiero.com/forum/Forum2/HTML/109173.html

valve cutaway.PNG (221.18 KiB) Viewed 901 times

in fact, in his thread he mentions that it does very little to save you in the event of a hydraulic failure, other than to make the illuminate the light on the dash. In my opinion, the light on the dash is more or less useless, by the time it comes on, you should already know there's some kind of problem with your brakes.

Here's another cutaway, this time the actual parts.

cvalve_name.jpg (37.39 KiB) Viewed 901 times

Based on what I see, and my understanding of hydraulic circuits, I would tend to agree that it doesn't provide much in the way of safety, other than the little light on the dash, which like I already said, it most likely coming on after you already know there's a problem.