Brianbajnuth

I have the front brake lines hooking up straight to master on the back port of the master cylinder. The rear brake lines are hooked up to the front port of the master cylinder and has a wilwood proportioning valve. For rear brakes I have disc (pbr brake calipers). How can I add more stopping power to the rear. I have the wilwood valve turned up all the way but still isn't enough power to lock the rear wheels.

RBob

iTrader: (1)

Double check the prop valve install, as for in/out and the adjustment. IIRC, turned fully in has the prop valve doing very little (most pressure to rear brakes).

If that isn't it then may need better brake pads. They cost more but do make a difference. StopTech makes a decent street performance pad at a reasonable price.

RBob.

Brianbajnuth

Ok I have the valve turned all the way up and it's very little stopping power.
I feel like the master cylinder is not giving the rear brakes enough pressure for it to lock up. I'm gonna attempt to tee the front brake line into the rear brake line before the proportion valve. That should give me enough power to lock up the rear brakes. Then I'll adjust it to my needs with the valve.

Does this sound good?

AlkyIROC

If the adjustable prop valve is wide open and isn't allowing enough pressure to the rear brakes then you can just take it out and try without it to see if you have enough brake pressure for the rear brakes.

You also need to look at basic hydraulic pressure principals. I'm not sure of the bore diameter of the master cylinder but lets use 1" for easier math.

Lets say you apply force onto the brake pedal and the brake booster pumps it up to 500 pounds pushing on a 1" piston in the master cylinder. That means you now have 500 pounds of hydraulic force everywhere inside the hydraulic brake lines. All that force is equal everywhere in the hydraulic system. Hydraulic pressure now pushes against caliper pistons. Lets say the front calipers have a surface area of 2 square inches. That means the 500 pounds of force is now applying 1000 pounds of force against the brake pads in each caliper. The distance they travel will be half the distance that the master cylinder travels. That same pressure is also going to the rear brakes but because most of the braking power is done by the front brakes, rear calipers have a smaller diameter piston. Lets say it's only 1.5 square inches. The 500 pounds of force is only putting 750 pounds of pressure against the pads.

That may be enough for the front brakes to do most of the work and the rear brakes work without locking up. Now you put better calipers on the car and have 2.5 square inches of piston. You now have 1250 pounds of force pushing on the pads and the rear brakes start locking up too soon so an adjustable proportioning valve is installed to limit the pressure going to the rear brakes.

That's why many performance brake systems use 2, 4 or 6 piston calipers. Combined, they provide a lot more force against the pads than a single piston will do. Another way to increase the force is to use a larger diameter piston master cylinder. Common sizes are 7/8" and 15/16" but the 1" and 1-1/16" provide more pressure behind the piston.

Any number of things may be causing poor rear braking. The proper way to test it is to have 2 pressure gauges installed in the bleeders and see if they have identical pressure front and rear when the brakes are applied. These pressure gauges are usually up to 1500 pounds. Other than that, it's really just trial and error to get front and rear pressures balanced properly so that all 4 wheels share braking equally especially under hard braking.

Or... as mentioned above, you may need better brake pads. Maybe the rotors are glazed.

According to the Wilwood instructions, when the valve is turned all the way out (counter clockwise), it is providing maximum pressure reduction of 57% braking power. As you turn the valve clockwise, you are increasing the braking power to the rear brakes. I think you may have it adjusted for maximum reduction.

Brianbajnuth

Very good explanation.
I believe teeing into my front brakes would solve the problem with the proportion valve. Let's see how it turns out

QwkTrip

iTrader: (22)

Darwin Award in the making on at least two levels here

scoflaw

iTrader: (2)

A smaller diameter master cylinder piston generates more pressure

Brianbajnuth

Do you know the stock master cylinder size?

Bob88GTA

iTrader: (2)

I'm gonna attempt to tee the front brake line into the rear brake line before the proportion valve. That should give me enough power to lock up the rear brakes. Then I'll adjust it to my needs with the valve.

Does this sound good?

I really don't think I personally would do that...........

Brianbajnuth

Care to explain?

Bob88GTA

iTrader: (2)

Brian
AlkyIROC just explained the physics of how braking systems work. You will STILL have the same surface area of the pistons to deal with. And, on a safety note.....if you somehow lose your front brakes, that are now connected to the rear, you have no brakes at all. Yes, I know.....Highly unlikely. I guess I'm too safety conscious.

I'll sit this one out.
Good luck

Bob

scoflaw

iTrader: (2)

Agree with Bob, the split reservoir is a saftey feature, break a line and you have no brakes. Not the way to go.

I am using a master out of a 95 car, fits perfect, line fittings are the same, and has the higher pressure small diameter piston.

Get on Autozones website, punch in different vehicles, and they will give you the specs your looking for

QwkTrip

iTrader: (22)

The master cylinder has two pistons that are in tandem, one for front brakes and one for rear brakes. One piston moves the other so for all practical purposes the front and rear line pressure is the same at master cylinder. A proportioning valve steps down the line pressure to the rear brakes. So you can see that connecting rear brakes to front brake line will not increase line pressure. In fact, it will LOWER line pressure because one piston in the master cylinder can't move the volume of fluid to operate all 4 brakes. The likely outcome is the brakes will either be very weak or maybe not work at all.


The front brakes should grab first. That is why the master cylinder has the front brakes on the primary piston, and the rear brakes on the follower piston. Swapping brake line positions at the master cylinder will cause rear brakes to actuate first, causing you to crash when the car does a flat spin on the freeway. This is ESPECIALLY RISKY with a stock 3rd gen master cylinder because it has a quick take-up piston that moves a lot of fluid to the front brakes upon initial movement of the brake pedal.

You think you want rear brakes to lock up but the rear brakes are intentionally designed to not lock up to promote predicable handling. If you tighten up the rear brakes to the point where the brakes lock up, then you run the risk of doing a flat spin.


Your brakes are probably fine. My guess is you have air in the line or a leak.

QwkTrip

iTrader: (22)

No, the master cylinder can't save your butt in that case because front and rear pistons are in tandem. Lose pressure in one chamber and you lose pressure in both chambers.... and no brakes.


It is the stock combination valve that saves your butt. It has the pressure switch in it that blocks off the failed line so you can have a chance to come to a stop. He eliminated the combination valve and replaced it with a proportioning valve that has no safety net.

QwkTrip

iTrader: (22)

I have the pressure curve for the Wilwood valve and I can tell you that the "100%" setting is straight through -- no pressure reduction, same as having no prop valve at all.


Again, I think you have air in the system and/or a leak in the rear somewhere.

scoflaw

iTrader: (2)

[QUOTE=QwkTrip;6090794]No, the master cylinder can't save your butt in that case because front and rear pistons are in tandem. Lose pressure in one chamber and you lose pressure in both chambers.... and no brakes.


/QUOTE]

I'll stand by my statement as being correct. Ever bled brakes? Notice how 1 reservoir can be empty while the other is full and working.

Aftermarket prop valves are not to be used along with the stock prop valve.

QwkTrip

iTrader: (22)

You're standing on quicksand.

That is true. Random piece of information that doesn't matter for this thread, but true.

scoflaw

iTrader: (2)

[QUOTE=QwkTrip;6090847]According to your previous post, it's the stock prop valve that's saving your *** not the master cylinder. Sooo do you think that the aftermarket prop valve manufactures would tell you to remove the stock prop valve if it's putting your life in risk ?

QwkTrip

iTrader: (22)

Your supposed arguments are irrelevant. The fact is you're avoiding having a technical discussion because you don't have a working understanding of the brake system.


You're the one that took a hardline stand with no supporting facts. I'm still waiting for your technical explanation.....

QwkTrip

iTrader: (22)

While we're at it, this is both right and wrong at the same time. A smaller bore diameter in the master won't change his line pressure, but it will change pedal feel.

A smaller bore increases pedal travel and decreases pedal force to achieve a certain line pressure. A larger bore can still achieve the same line pressure but the driver has to push harder on the brake pedal. The bore diameter should be chosen to create the pedal feel the driver likes. Also, care must be taken when going with a smaller bore diameter to make sure there is still sufficient fluid volume to operate the brakes because you can literally run out of pedal travel.

3rd gen brakes are pretty soft from the factory. Usually people prefer a firmer, shorter brake pedal which is achieved by using a master cylinder with a larger bore diameter.

scoflaw

iTrader: (2)

Let google be your friend to research this for yourself. At 65 years old, I think I know a thing or 2 about brakes. You've made 2 erroneous statements.

1] If you lose a line to 1 reservoir, both reservoirs will lose fluid because the pistons are in tandem

2} The stock prop valve is what saves your butt, it has a pressure switch built in to shut down the bad line..

QwkTrip

iTrader: (22)

Still no facts. Still no explanation.
Stop posting if you're going to have such hollow content

FYI - if you want to compare paper credentials then you'll receive a major beat down. But this isn't about credentials - never should be. This is about being able to articulate facts and data that can stand on its own. But you've got none of that, do you?

Bob88GTA

iTrader: (2)

scoflaw

iTrader: (2)

This will be my last post, getting no where disputing this with you. Soo why don't you post up some facts to back up the 2 items I listed in my last post.

While your at it, post up those paper credentials of yours. Mmmm. going to eat some popcorn and drink some beer while I wait.

To the OP, sorry your thread got derailed a bit. Do some research and good luck.

QwkTrip

iTrader: (22)

Nice try. The deal here is that you explain yourself. If you need a good example what that looks like then you can refer to my posts.

Now there you go changing my words. Shame on you. I said, "Lose pressure in one chamber and you lose pressure in both chambers", with the chamber being master cylinder bore within context of that post.

If you lose a brake line with an aftermarket prop valve then you will lose all brakes. If the leak is small enough then you will have some limited braking capability until the fluid is pushed out of the line, and you will have to keep pumping the brakes to keep building up some line pressure.

Oh, my gosh, you still don't get it. At a site like this it doesn't matter who you are, it only matters if your words can hold their own weight.