Hot Rods Question about a brake proportioning valve

Can you have the valve but no switch?

 

i remember the first time i read [here a bunch of years ago] that there was no shut off capability on those blocks i thought it was strange as i have experienced all the symptoms and remedies of them shutting off. for example, a plow truck would come in to my shop after plowing all night after a storm. sometime in the middle of the night one of the brake lines had rotted through, but they just kept plowing. [snow banks stop plow trucks just fine]. we would replace the lines from the block back and couldn't get it to bleed. remove the new line from the block and plug off the outlet. pump the pedal like prom night, reconnect the line and gently bleed the brakes and it would work. not once, not twice but a couple dozen times.

 

A educated foot can feel when the valve over centers.

 

I’ll repeat my question then....if that valve shuts off the side that leaks why do we need a tandem master cylinder?


Sent from my iPhone using H.A.M.B.

 

Asked and answered

When a bleeder is open the pedal goes to the floor does it not? when a line is open the same condition, pedal to the floor yes? However when that valve is present and the shuttle has shifted you will have a pedal that DOES NOT go to the floor.


And ...
Because it's a dual or tandem master there are 2 sides, a natural consequence.
Only Because there are 2 side can there be a pressure differential between the 2.

 

With a tandem master no the pedal does not go right to the floor, the purpose is to have a fail safe, or two entirely independent hydraulic systems. If we have a leak in one side the other side still functions. By side I’m referring to front or back unless it’s a diagonally split system. By your description we should not need a tandem master. Look up the function of a tandem master cylinder and you will see that it is the failsafe in the system, hence the reason we no longer have emergency brakes and now have park brakes.

Bruce’s problem sounds like a bad secondary seal on the piston inside the master cylinder that is for the rear brakes, not sure if it’s the primary or secondary in his application. It allows fluid to p*** not building pressure in that half of the system.

Sent from my iPhone using H.A.M.B.

 

You can argue all you want. With as many folks as you want. If you feel you're right you should hold your ground and stand up for it.

On a system without the valve,,,
What happens when you have a great high pedal and then open a bleeder with foot pressure?
The pedal goes down, and down and down and down to the floor. The other side still works and has much less pressure, especially pedal feel,
The same thing happens with a line failure, the pedal level drops and drops and drops to the floor.

With that valve the pedal gets a little wired feeling but it does not drop.
If the shuttle is shifted its nearly impossible to bleed the brakes. You open the bleeder, expected the release of fluid and the pedal to drop but it does not. You'll get a few drops and no matter how hard you push the pedal will not drop nor will it push fluid. And that's why it's nearly impossible to bleed the brakes. If it's not pushing fluid it won't push air nor build pressure.

If you have the valve you can take the front caliper off leave the lines open and drive the vehicle on fully functional rear brakes. Or remove the rear wheel cylinders and have fully functional front brakes, neither one will allow the pedal to fully drop. You can't do that on any system without the valve, the pedal is on the floor.

Also, without the valve and a broken line you can fill the master and pump the brakes and large amounts fluid will clearly indicate where the line break is. With that valve you can't do that, you'll get a few drops and have to look for the break closely to find it.

 

And the reason you can do that is the master cylinder not the magical valve, however I will gracefully bow out as arguing will get no where. I am confident in my knowledge of how brake systems work and that is enough for me, I don’t need to prove it to everyone to feel important, I thought I could add to this thread but apparently cannot so I will move along. I am not just a back yard mechanic who is taking a shot in the dark on this stuff either....


Sent from my iPhone using H.A.M.B.

 

Get a valve just like that and rebuild it.
Or call the guys who put out the video, let them know it works.

 

Ok thanks for the advice, I may just do that


Sent from my iPhone using H.A.M.B.

 

If you want to know if the shuttle shifted put a test light on it, Hook it to + on battery touch the point to the pin at the valve , light on means it moved ,no light did not move.

 

Thanks all of you Yes it seems like the wagon was setting a while I am like a magnet that draw all wagons that sat a while to me LOL .I am going to see if I can order a new valve .Mind as well do it right since I am replacing all the brake stuff.While I was bleeding the rear brakes I noticed the rear brake line that ran across the rear pumpkin is rusty too. As soon as I get the valve I will replace that brake line too. Funny the station wagon it self has no rust on it! Thanks Bruce.

 

I had a couple of GM wagons of that era... rotted out brake lines were the norm on them. So were fuel lines, that should run down the p***enger side ch***is rail.

 

Heres a pic of the proportioning valve, the 'shuttle' is sealed from fluid, if fluid from front or rear circuit enters the chamber it'll leak past the switch itself. If the 'shuttle' is driven one way or the other there is no effect on the stopping ability or fluid transfer.

 

My reading indicates there are two types, those that do and those that don't:

Pressure Differential Switch
All modern vehicles are equipped with a red brake warning light. One of the purposes of this light is to inform the driver of either a fluid loss or pressure loss condition. Fluid level switches are used for fluid loss and pressure differential switches are used for sensing pressure. Pressure differential switches use a piston that is exposed to both the primary and secondary hydraulic circuits as in Figure 19. The pressure differential switch consists of the piston, electrical contact switch, wiring and warning light.

If the vehicle experiences a hydraulic leak or failure of either the primary or secondary half of the master cylinder, a difference in circuit pressure will be created. The pressure from the functioning portion of the hydraulic system will force the pressure differential piston toward the side with the pressure loss. The movement of the piston will cause the electrical contact switch to go from open to closed as in Figure 20. This will complete the ground circuit to the red warning light and will illuminate it.

​

The pressure differential switch piston never moves until a pressure difference occurs. The bore in which it travels becomes corroded and tarnished over time. When the piston does move, it does not always want to return after the necessary repairs are made. The most effective method to re-center the piston is to repair the failure, bleed the system and then create a pressure loss opposite what moved the piston in the first place. If the original failure was a broken rear brake line, fix the line and bleed the rear brakes. Next open a front bleeder and spike the brake pedal a couple of times taking care not to exceed half pedal travel. Tapping on the side of the valve with a rubber or br*** hammer can also aid in the process. If after using this method the piston still won't re-center, then the combination valve should be changed.

​

Special Purpose Pressure Differential Switch
There are a number of pressure differential switches that perform a second function. In addition to completing the ground to the red warning light, they also shut off the fluid flow to the rear brakes if a rear hydraulic failure has occurred. The fluid flow to the rear brakes is through the ports in Figure 21. If a rear pressure loss occurs then the pressure differential piston moves toward the rear brake circuit and closes off the fluid p***ages to the rear brakes as in Figure 22. When dealing with this type of valve you must re-center the valve before the rear brakes can be bled. Bleed the lines at the master cylinder and the fitting on the valve. Open a front bleeder and spike the brake pedal a couple of times taking care not to exceed half pedal travel. This usually will re-center the piston. If it won't re-center, the combination valve will have to be replaced.

http://www.studebaker-info.org/Tech/brakes/bf50014.html

 

57 Fargo: My first car was a 63 Falcon, brakes worked fine until I was pulling up to the curb at the downtown hardware store where the "fruit jar" master cylinder failed without warning. The dual master cylinder was created to (hopefully) give you some warning of impending brake failure.
Bruce: My truck sits for weeks at a time and developed a leak at the rear of the master cylinder. When it was time to bleed the "rebuilt" master cylinder I got from the parts store, the front bled fine, the rear, all I could get was a dribble of fluid. I exchanged the master cylinder with one that actually worked and all was fine.

 

Cracker head.That's the plan, replace everything that's rubber, going to get the radiator flush out too.Thanks Bruce.

 

Bruce, Saw this add today and thought about you. Louisville, Ky. craigslist #6514924748; 1954 Buick station wagon. 58,000 miles---$6000. Don't know if you have bought another car yet, or if you know about this one, but it has pictures of the car and you know how we all like to look at pictures. Stay safe... Ray.

 

Thanks 50 customcoup, I did aready buy a wagon.Plans are to make this one road worthy new paint, wood grain and sell it to get an older station wagon.Thanks Bruce.

 

Same on newer GM vehicles. Brake and fuel lines always rust out. It wasn't as bad when mechanical pumps ****ed fuel as the vehicle might quit running and have a gravity leak. Having a fuel pump in the tank providing 60psi means a high pressure stream from a leak.

It doesn't do any good to argue about the multi-function brake valve. It brings up the same problems as politics and religion. You can't change anyone's beliefs. We learned about them in auto shop cl*** in 1970. There was a film supplied by GM from the 1960s. It was about the same as the posted video.

The reason for the dual master cylinder is that there can be small leaks that don't cause a pressure drop but do cause fluid loss over time. The master cylinder can also leak.

 

Well going to look for a brake proportioning valve Friday at a pull apart yard. They have a lot of good clean late model cars there. Both radiators shops I used to go to had closed down. Found a huge one in Knoxville about a 45 minute ride each way from my house. Guys were very friendly and said they would start by letting my radiator soak over night and see if they could work with it.Bruce.

 

Bruce, i don't think all proportioning valves are the same. [someone may chime in] did you try my "trick" to center it? if not, do that and check for a collapsed line first before buying one.

 

Correct. There were many proportioning valve operational specs, depending on the vehicle; engine, tires, wheel base, CG, weight, brakes etc.

 

I’ll leave the combination valve issue alone this time but the purpose behind the tandem master cylinder is to have two independent systems to prevent total brake failure in the event of a leak, ANY leak will cause a pressure drop.


Sent from my iPhone using The H.A.M.B. mobile app

 

Thank you, I hope Bruce will listen to you and not blindly follow some of the advice given.


Sent from my iPhone using The H.A.M.B. mobile app

 

Hey Bruce,
Not many late model cars use those valves anymore, most of the time they are part of the ABS block. Cars I used to get them from were 1978-1988 G body monte carlos and their clones, or if you put a "T" on the output you will find different style aluminum ones on 90s S10, caprices, and other GM cars. The aluminum blocks only have one output for the front and one for the back, and the inlets are switched around. Get a good look at whatever valve you use to see which ports are for the front brakes and which for the rear. Newer 90s blocks also use metric nuts and bubble flares.
When all is said and done I do not get the used ones anymore, new are plenty cheap for either drum/drum, drum/disc, or disc disc. This one is for disc up front and drum rears.

https://www.summitracing.com/parts/clp-pv-2

Also before you condemn the valve disconnect the line at the valve to make sure it really is plugged off. I have had rear brake hoses plugged solid from degradation.

 

Some said to disconnect the back line and stomp on the brake to repostion the P.V. I did that and the whole works shot out of the valve. It looks pretty worn so I am going to match one up Friday when I go to pull a parts. Thanks Bruce.

 

sounds like you unscrewed the large nut on the back, just needed to do was disconnect the brake line. Good luck.

 

Try to find a valve from the same size GM wagon if you want it to work correctly. They are calibrated for the weight and distribution of the car they are made for.

 

You are right, I screwed up.LOL.Bruce.