Braking question

[97JadeTJ]

So today I attempted to stop at a stop sign and the brake pedal went to the floor… was able to get it stopped and made it to the house to find that my right front brake hose had a hole in it from the tire rubbing… got a new one and installed it and good to go… my question is shouldn’t there have been some find of braking coming from the rear brakes… isn’t that the purpose of combination valve?

 

[1997JeepTJ]

This is a simplistic reply.....brakes require hydraulic pressure to work and the hole in your front brake hose didn't allow that pressure to build.

 

[97JadeTJ]

should the proportioning valve not shut down the compromised area i.e. The front?

 

[1997JeepTJ]

No, that's not how the prop valve works. It's job is to apply a percentage of the braking force to the rear brakes.

If the braking system is compromised by low to no fluid it does not work effectively or perhaps at all as you discovered.

 

[macleanflood]

You'd only see that on a dual pot brake system.

-Mac

 

[97JadeTJ]

I understand their was a shuttle behind the brake switch on the valve that stayed in the middle unless there was a compromised area and it slid forward or backwards so you would have some braking…

 

[97JadeTJ]

https://www.jeepforum.com/threads/97-tj-brake-proportional-valve-problems.1286157/post-12475722

@mrblaine

Underneath the master cylinder there is what is known as a combination block. In that block besides the obvious splitter to the front and rear hydraulic circuits, there is a shuttle valve just inside the brake light switch that will close off the front or rear circuit if differential pressure goes high enough. It does that in the even of a catastrophic leak. The front portion of the combination block houses the proportioning valve to the rear brakes. It's underneath that hex port with what looks like a large bleed screw under the rubber cap. Inside of that is a plunger with a cup seal and a spring that does the proportioning.

so why would mine not shut off the front circuit?

 

[mrblaine]

https://www.jeepforum.com/threads/97-tj-brake-proportional-valve-problems.1286157/post-12475722

@mrblaine

Underneath the master cylinder there is what is known as a combination block. In that block besides the obvious splitter to the front and rear hydraulic circuits, there is a shuttle valve just inside the brake light switch that will close off the front or rear circuit if differential pressure goes high enough. It does that in the even of a catastrophic leak. The front portion of the combination block houses the proportioning valve to the rear brakes. It's underneath that hex port with what looks like a large bleed screw under the rubber cap. Inside of that is a plunger with a cup seal and a spring that does the proportioning.

??

 

[97JadeTJ]

??

97 tj… front right brake line broke which left me with the pedal to the floor and unable to stop…should my rear brakes have held… trying to determine if there is something wrong with the combo valve…

 

[mrblaine]

97 tj… front right brake line broke which left me with the pedal to the floor… should my rear brake have held… trying to determine if there is something wrong with the combo valve…

You generally have to pump the pedal to shift the valve. Try that and see if the dash light comes on.

 

[97JadeTJ]

You generally have to pump the pedal to shift the valve. Try that and see if the dash light comes on.

I pumped it several times and the light never came on..

 

[hear]

I'm with OP in sprit here. I thought the purpose of a dual chamber master cylinder reservoir was that 2 wheels were on one brake circuit and 2 were on the other, so if there were one or more leaks on one circuit, the other circuit would maintain pressure to maintain braking capabilities. Whether that was through a prop or combo valve, I have no idea. Maybe I've been wrong all this time.

I think back to the time the caliper came off my JK in the Arkansas "mountains"....I didn't have any brakes, or at least very little braking power. Not enough to stop anyway. I've been thinking about this ever since that crazy night. Maybe it'll get resolved right here and now.

 

[TheBoogieman]

You'd only see that on a dual pot brake system.

Why do you need to bring pot into the discussion?

 

[mrblaine]

I pumped it several times and the light never came on..

Does the brake light on the dash turn on when you apply the parking brake?

 

[mrblaine]

I'm with OP in sprit here. I thought the purpose of a dual chamber master cylinder reservoir was that 2 wheels were on one brake circuit and 2 were on the other,

Sorta correct. Take the reservoir out of the above sentence and it is more accurate.

so if there were one or more leaks on one circuit, the other circuit would maintain pressure to maintain braking capabilities.

Incorrect. If that were true, you would never be able to bleed your brakes without opening at least one bleed screw on each axle.

Whether that was through a prop or combo valve, I have no idea. Maybe I've been wrong all this time.

You've been wrong all this time. There is not really a combo "valve". The collection of several functions into one block so they are combined lets us generically refer to the combination valve or block under the master as such. In that block are the things stated above, one of them is the shuttle valve which is designed to be shifted by differential pressure. The proportioning valve function is not part of that and is not connected.

I think back to the time the caliper came off my JK in the Arkansas "mountains"....I didn't have any brakes, or at least very little braking power. Not enough to stop anyway. I've been thinking about this ever since that crazy night. Maybe it'll get resolved right here and now.

Take a picture under the hood in the area around the master and booster in your JK. Study it a fair bit and then draw an arrow pointing to the combination block similar to what the TJ has. I can save you some time though, it isn't there. The JK has 4 wheel abs and in your scenario, I'm guessing that the system fully freaked out since it would be trying to apply more brake pressure to the swinging caliper only to have it not work and slow down that wheel.

Someone more versed in what the JK system would do in that case will have to help out here.

 

[97JadeTJ]

Does the brake light on the dash turn on when you apply the parking brake?

No

 

[mrblaine]

No

You should fix that.

 

[97JadeTJ]

That’s my first chore of the day.. Thanks

 

[97JadeTJ]

i Can’t seem to figure out why the brake light won’t come on…it should light up on ignition switch but it doesn’t… the parking brake switch works, tested with a multimeter… the combo valve brake switch works, plunger in causes resistance and out is open… shuttle valve is in the middle, I’ve got front and back brakes… bulb is good, I took the cluster out and checked the bulb…fuse 10 is good…. This started because my front right brake line was compromised, the brake pedal went to the floor and the brake light did Not come on… what am I missing?

 

[NskLJ]

i Can’t seem to figure out why the brake light won’t come on…it should light up on ignition switch but it doesn’t… the parking brake switch works, tested with a multimeter… the combo valve brake switch works, plunger in causes resistance and out is open… shuttle valve is in the middle, I’ve got front and back brakes… bulb is good, I took the cluster out and checked the bulb…fuse 10 is good…. This started because my front right brake line was compromised, the brake pedal went to the floor and the brake light did Not come on… what am I

I used the 98 diagram but it should be the same.
Disconnect the switch connector at the brake valve under the hood, turn key on and ground the gray with white stripe wire (blue line) if the brake light does not turn on. You have an open circuit from the brake switch to the bulb. If it does turn on connect the two wires together and test while starting and with the parking brake if the light works now the switch in the brake valve is bad.