Technical Master Cylinder Issue

Hey all..I have an odd issue trying to bleed my brakes and I am hoping you guys might help me come up with a couple more ideas for troubleshooting.

The system is a simple front disc rear drum system witha dual 7" brake booster and a 2 resevoir master cylinder the resvoirs are the same size. I have the front resevoir going to the front calipers with a wilwood 2 psi proportioning valve and the rear resvoir going to the rear drums with a 10 psi proportioning valve.

When bleeding the rear and I crack the bleeders the pedal goes to the floor as it should.

When I attempt to bleed the front, the pedal Only drops about an inch and wont go to the floorlike it should.

I opened the fitting right at the front resvoir of the master cylinder and the same thing happens so its somethign within the booster master cylinder area..but I cant imagine what...If the pedal goes all the way to the floor when doing the back breaks. It should go to the floor when doing the front or am I wrong in this thinking?

Thanks!

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Prop valve putting pressure to the back?

Are you getting any fluid out of the front bleeder?


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Yes I do until the pedal stops


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Check valve in the correct way?

 

I had a miss adjusted manual proportioning valve unbeknownst to me that drove me and several friends crazy for a few weeks. What it did was put to much to the rear so the shoes expanded out to the drums stopping the pedal from traveling far enough to push the rod into the master cylinder to activate the other chamber for the front brakes.

 

I double checked the valves and they are facing the right way...they arent manual portioning valves...

However the lines are not tied together and the resevoirs are separate can one side truly effect the other?

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Proportioning valve may tie the two circuits together. Also the safety of a two circuit system, OEM style, has a valve to cut off an open circuit and route pressure to the other one.



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Pics please,,,if possible ?
And when you say front reservoir,,,,do you mean front toward radiator,,or front toward the firewall ?

Tommy

 

The proportioning valve absolutely does not tie the two systems together, and pressure is not routed to another circuit in the event of a failure. The whole point of a tandem master is to have two separate systems, they are not tied together in any way


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Heres some pics..

So the master cylinder is under the cab of the truck and the master cylinder booster combo is facing toward the front of the truck. In the pic that booster is toward the front..So when I say dual resevoir I got a single master cylinder with 2 reservoirs..simple plumbing where a single line comes from the front res into a 2psi valve and the rear res goes to another valve. So unkess I misunderstand the workings of a master cylinder the front circuit and rear circuit are separate...

When I bleed the rear the pedal goes to the floor as it should

When I bleed the front calipers I push the pedal and,then crack a bleeder and the pedal Only goes about an inch and stops...fluid comes out of the front bleeder until the pedal stop but I still got anither 2 to 3 inches before the pedal would hit the floor.

Now in writting this I should mention this MC was on this truck without a booster and worked fine...in rebuilding the truck I added the booster...now it was under the floor as well but never paid attention to which res was plumbed to which side of the vehicle. Could this MC need the rear res plumbed to the front as if it were mounted on the firewall?



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What calipers on the front?


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A couple of things. If it were me I would try to replicate how it was installed in the truck. Also did you fabricate a pushrod or use the one from the truck? Any chance it is simply too short? Also if I recall correctly the rear chamber of the brake reservoir goes to the disc or front, and the front chamber one goes to the rear or drums.

 

Unless it’s a quick take up master, it makes zero difference which reservoir goes to which end...I wish people would quit saying it does. If there are different size reservoirs then the bigger one will go to the discs, only because of the increased volume required for wear compensation, as far as function, Zero difference


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Just trying to help out and offering information from what I have read or understand. If it makes zero difference help me understand why most of the major car companies do it the way they do?

 

No idea why it’s generally done that way. But I can assure you it will work either way. Same as people being worried about whether it’s a disc or drum master, what’s important is the bore size. As I said reservoir size is important however if one makes sure it doesn’t run out of fluid as the brakes wear then no big deal. There are lots of things in our industry that have been perpetrated over and over again. Especially brakes.

Just to clarify I wasn’t trying to pick on anybody, also just trying to clarify things.


I still would like to know what calipers, before I offer a theory.




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They are metric calipers purchasd from jw rod garage as the whole front ifs is a jw rod garage with power syeering and 11" discs..exact car they came off of I believe they are similar to s10s..

The booster and pedal assembly was just a cheap 54 chevy truck setup that replaces all the original pedals and bolts int he stock location. The booster is mounted where it was originally when I bought the truck it just didnt have a booster..i had to add the spacer in the master cylinder so the booster rod would compress the MC piston...

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Now where this issue lies is that the pedal travels the full distance of the MC piston as proven when I bleed the back the pedal goes to the floor..but when I try and bleed the front it doesnt travel as far about 2" less actually.



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Might verify the master is fully retracting, allowing the piston to re-position for a full stroke. Possibly the booster link is too long and is not allowing a complete stroke of the master piston to activate the front circuit completely.

 

Basically it is sounding like I need to take the MC out of the truck and start measuring all the push rods and clearances.

I did unbolt the the booster rod from the pedal and I know it is all the way out but I have not measured the throw of the rod on the other side of the booster going into the MC at least not while its mounted in the truck...i didnt have a floorboard to contend with.

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Why do 95% of brake bleeding and pressure issues I see here involve a Corvette style master? Just an observation.

 

If the pedal goes to the floor when he bleeds the back then it’s not a lack of stroke in the master. Let me think about this for a while.


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Mmmmmnn.

As stated by Gordon, the rear chamber is the primary in most factory applications. There’s probably a reason for this. Maybe we are seeing it here.

Will the cylinder work well plumbed front-front rear-rear (like you have it) rather than front-rear rear-front (like factory applications)?

Theoretically it should work if both chambers are the same size.

Keep in mind that working/functioning in a custom application is a lot different than functioning correctly as per factory design.....

My question is, do the circuits work progressively?
Meaning.... does the primary (rear of the master) activate first then followed by the secondary circuit ( front of the master)?
This would be piston/port design rather than reservoir chamber size.

Does this really matter in a custom application if both reservoir chambers are the same size? I know it does on a disk drum master with a larger rear disk reservoir. Those masters come from vehicles that used calipers requiring more fluid and that may not be needed in a custom application. Again port progression may be a factor... or not.

On stuff like this I like to know how the system was plumbed on the donor vehicle the master came from. Then I would plumb it in the same manner.

The elephant in the room.....
One would think that on a truly dual circuit, the pedal should never “ fall to the floor” when you bleed brakes.
Why?
Because the circuits are...separate. When you bleed one circuit the other still should be functioning. Correct?

Remember it’s a dual circuit master, not a single. In theory one circuit should always be working.
If the front blows a line the rears should function and vise versa.

So the pedal falls to the floor when you bleed the rears. The pedal does not fall to the floor when you bleed the front......

I do believe the rears are on the primary.

Maybe my ramblings are “mental masturbation”.
Before I did anything else I would get all four wheels up. I would bleed the system with an assistant.......With the rear brakes bled....pedal to the floor as you say......I would try to turn the front wheels to confirm that the dual circuit feature is actually working. Then I would do the same for the fronts. You need an assistant because “to the floor” and “almost....too the floor” requires that input from the driver. The primary circuit may seem to be on the floor but really not. This is why you need an assistant in the cab and why you need to turn some wheels. Do a real function test.

 

Being as the front circuit is plumbed to the forward reservoir....if the booster link is too long it will allow the rear circuit to function, but will not allow the front circuit to get a full stroke. Most likely if the lines were reversed the rear brakes would do as the fronts are doing now.

 

Without knowing what parts he has installed, and no pictures, you could be right, or wrong.

A GM “proportioning valve”



most certainly does tie front and back together, with the sliding piston valve used to cut off pressure to a failed circuit, allowing the non failed circuit to work. At least in theory. Having experienced several single circuit failures, the other circuit “work” is definitely not ideal.

He may have something that is plumbed in only the rear circuit. Without pictures, it’s impossible to know.



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I just performed this operation on my truck. My setup is a corvette style dual pot master (1” bore size?) with a reproduction GM combination valve made for drum/disc. A little different but it is 10psi/2psi.

The front of my master plumbs you the rear of the combination valve and rear of the truck. The rear of my master plumbs to the front of combination valve and the front of the truck.
It was a real pain in the butt to bleed it by myself but I managed. I was losing pedal in the reverse of what you noticed.

Hopefully this will help you.

 

Pop the cover off. Are both reservoirs the same size? If one is larger, that one goes to the front disks.




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Same reason every SBC on Craigslist is out of a Corvette.



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I have had both front and rear circuit failures, in reasonably modern, factory stock cars. The pedal does indeed go to the floor. You get some pressure on the remaining circuit, and can use it to gradually slow the car, but it’s nothing like normal braking.



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Sorry I’m right either way, no where are the hydraulic circuits connected. I’m not getting into the combination valve operation. The purpose of having two separate circuits is to always have one working.


And for the last time, it makes zero difference which end is hooked to which end of the vehicle unless it is a quick take up master. Both circuits apply at the same time. There is no delay in the master cylinder. That is why there are metering valves and proportioning valves. The pressure differential switch is also a part of the combination valve, it senses a pressure difference and turns the light on.

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