Brake master cylinder riddle

[DC_Gearhead]

So I got a new proportioning valve I’m about to put in to replace the stock brass one. Im disconnecting my lines from the MS and the back line drained lots of fluid that I caught with a rag. Lots equals about a coke cap full. I loosed the front one and nada.

What gives? I was having a hard time getting pedal and figured the proportioning valve was bad. Any why it would be wet and dry? MS is full still.

Facts:
Drum rear
Disc front
Power 7”
Don’t remember where the MS is from

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[DirtDonk]

It's a CPP brand, so aftermarket, but could come from just about anywhere. Does it have ports on both sides (other side plugged) that are the same size?
The dry port could absolutely indicate an issue, but it could also be perfectly normal. You don't always get much fluid out, but if it's bone dry inside then that would indicate to me at least that something is amiss inside the master.
What happens when you push the pedal now?
Obviously with a rag/container at the opening and don't push the pedal far. But it would let you know whether or not something's up.

paul

 

[DC_Gearhead]

It's a CPP brand, so aftermarket, but could come from just about anywhere. Does it have ports on both sides (other side plugged) that are the same size?
The dry port could absolutely indicate an issue, but it could also be perfectly normal. You don't always get much fluid out, but if it's bone dry inside then that would indicate to me at least that something is amiss inside the master.
What happens when you push the pedal now?
Obviously with a rag/container at the opening and don't push the pedal far. But it would let you know whether or not something's up.

paul

So Paul …….I just noticed my issue? My lines were plumbed backwards. Front to back ……back to front

Probably my issue since the backflow preventer would be backwards?

Thoughts?


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[DC_Gearhead]

Since it sounds like you have the lines disconnected at both ends, make sure you can blow air through both of them. If something is blocking the front one that might have blocked the master from flowing much out.

Which method did you use to bench bleed? The capped method, or the hose-to-the-reservoir method?

Paul

Hose bled it and yes it has caps both sides. Bench bled twice because of the weird pedal.

Back brakes worked great but nada on front.


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[Pa PITT]

SOME ONE ELSE Chime in on this.
I'm thinking the front bowl goes to the rears brakes & the rear bowl is for the Front.
So are you sure their plumbed wrong .

 

[DC_Gearhead]

SOME ONE ELSE Chime in on this.
I'm thinking the front bowl goes to the rears brakes & the rear bowl is for the Front.
So are you sure their plumbed wrong .

My assumption is that the bowl closest to the mounting end is the primary/front bowl. If that is correct, then I am backwards.

Which is exactly what you are saying too I think. Bowl closed to front of Bronco is for rears.

But yeah I’d love some confirmation


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[DirtDonk]

Must be something in the way I was doing it with my long answer. Maybe it’s that error message I used to get on the computer but on my phone it only says “give up dummy!”

 

[EPB72]

My assumption is that the bowl closest to the mounting end is the primary/front bowl. If that is correct, then I am backwards.

Which is exactly what you are saying too I think. Bowl closed to front of Bronco is for rears.

But yeah I’d love some confirmation


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That master is based off the corvette/camero master cylinder and a lot / most brake kits use it..In the OE use the front Port went to the front and the back port went to the back....when you buy the kits being CCP,Speedway,Jegs,and summit ,,with the master .prop valve and interconnect lines it's plumbed this way as well ...some fords and in our broncos case the stock routing was the opposite ...

I install them the GM way front to front ,,back to back..

 

[Apogee]

X2 on what EPB72 said, that CPP master cylinder (MC) is basically a 1968-1976 C3 Corvette unit hydraulically speaking. As such, it is a disc/disc unit without any residual pressure valves, and should be hooked up front/front and back/back. CPP offers that particular model MC in a Ø15/16", Ø1" and Ø1-1/8" bore size, so you would need to measure the bore in order to determine what you have, as I do not believe they externally mark or identify them in any way.

If it were mine, I would purchase some inverted flare plugs and then plug both outlet ports and test the MC. CPP offers those MC's with either 2X 3/8-24 inverted flare outlet ports or 1X 1/2-20 and 1X 9/16-18 inverted flare, though based on the picture I would guess yours is the former, not the latter. By "test the MC", I mean that I would fill the unit with fluid and then basically perform a plugged-port "bench" bleed on the EB by slowly stroking and releasing the pedal until the pedal is high/firm and hydraulically locks with minimal travel. Once you've achieved that, it means there's no more air in the MC and now you can firmly apply the brakes for a period of time and test whether the pedal sinks over time or not. If it does, then that means you're internally bypassing fluid past the piston seals, and the unit needs to be either repaired or replaced. If it doesn't sink over time, then connect the rear brake circuit and bleed normally until you have a good pedal, then repeat with the front brake circuit. This will minimize the number of variables in play at any given time and help you isolate and issues should you encounter them.

Tobin

 

[DC_Gearhead]

X2 on what EPB72 said, that CPP master cylinder (MC) is basically a 1968-1976 C3 Corvette unit hydraulically speaking. As such, it is a disc/disc unit without any residual pressure valves, and should be hooked up front/front and back/back. CPP offers that particular model MC in a Ø15/16", Ø1" and Ø1-1/8" bore size, so you would need to measure the bore in order to determine what you have, as I do not believe they externally mark or identify them in any way.

If it were mine, I would purchase some inverted flare plugs and then plug both outlet ports and test the MC. CPP offers those MC's with either 2X 3/8-24 inverted flare outlet ports or 1X 1/2-20 and 1X 9/16-18 inverted flare, though based on the picture I would guess yours is the former, not the latter. By "test the MC", I mean that I would fill the unit with fluid and then basically perform a plugged-port "bench" bleed on the EB by slowly stroking and releasing the pedal until the pedal is high/firm and hydraulically locks with minimal travel. Once you've achieved that, it means there's no more air in the MC and now you can firmly apply the brakes for a period of time and test whether the pedal sinks over time or not. If it does, then that means you're internally bypassing fluid past the piston seals, and the unit needs to be either repaired or replaced. If it doesn't sink over time, then connect the rear brake circuit and bleed normally until you have a good pedal, then repeat with the front brake circuit. This will minimize the number of variables in play at any given time and help you isolate and issues should you encounter them.

Tobin

Tobin this information above is exactly what I needed. I’ll update with findings.


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[DC_Gearhead]

So….

I removed the MS to check bore. 1-1/8” bore and same size outlets on both size 3/8-24 inverted flare. I reinstalled it with plugs and built up a good pedal with bubbles coming out of front reservoir. The bubbles kept happening at each pump until the pedal was fully pumped up high/firm. Only a couple of bubbles came out of the back reservoir during the entire activity. No leaks I could see coming out of MS. I let it sit for 30 min and came back. Pedal was different so I slowly pushed it again to see what happened. More bubbles in front with every pump.

The one with bubbles is the same reservoir that didn’t have and fluid come out when I removed the line. It is also the one I connected the front brakes to and couldn’t get pedal or really any front brakes.

Looks like I need a new MS. Correct me if I’m wrong.

Also, is it general agreement that there is no need to worry about the residual pressure valve with a disc/drum setup, right? Basically no need for a backflow preventer at all.


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[DirtDonk]

Generally that’s correct.
The two purposes of residual pressure valves are:
1. To keep a little pressure on the seals in a drum brake wheel cylinder. Only certain types of wheel cylinders require this I believe, and I don’t think ours are of that type.
2. To keep fluid from flowing back into the reservoir when the master cylinder is mounted low. Most vehicles, including broncos have a high mounted master cylinder so there is no chance that gravity will revert fluid back into the master cylinder and away from the brakes. Unless you’re parked on a very steep hill.

Ultimately drum brakes can only pull back as far as the anchor pin on the backing plate. So there is no danger of the springs pulling the shoes out of adjustment for lack of an RPV. As long as the shoes are adjusted properly in the first place this is not an issue.
Keeping some residual pressure on the seals in the cylinders is arguably a good characteristic. But most of us have been running around without an RPV in our systems for the last 30 years with no ill affect.
That we know of!

So yes, it’s probably not a bad thing to design your full brake system with an RPV in each circuit, but no you will probably not notice the difference if you do not have them.
And back to your original question, I was wrong saying that there was no backflow preventer. That is exactly what an RPV is, and our original drum brake master cylinder‘s from the factory did include them. Replacements on the other hand have not included them for probably the better part of 30 years. About the time they probably realized that none of the manufacturers were mounting their master cylinders down low on the frame anymore, so they could save a few million pennies by not including them.

 

[Rustytruck]

You want a residual pressure on the drum brakes as you release the brakes the pressure in the system drops. the rear brake cylinder is forced almost empty by the springs in the brake shoes. the pistons retract as far as they can go. the front releases pressure but don't retract so the system stays full not pressurized. now you step on the brakes the calipers close immediately and build pressure. but the rear drums are not pressurized yet because they are collapsed and empty. the travel of the pedal is trapped by the the pressure in the master cylinder and the front disc. this pressure should push the brake warning light on as one side has pressure and the other side doesnt. a good residual valve will keep the drum brake cylinders full.
do people get away with it? Yes they do as long as the master cylinder bore is large enough to over come the fluid loss before everything is pressurized.