Master Cylinder Plumbing

[slowcpa]

First time working on brakes so a little nervous. Purchased my 74 about 3 miles ago with a soft brake pedal. the po had updated the fronts to disc brakes and i found a small leak in the proportioning valve rubber port (on the underside of the rear post proportioning valve line). just to be safe, I replaced the valve and connected everything where it was, or so I thought. he (or maybe I) ran the large reservoir to the rear channel/circuit of the prop valve and the small reservoir to the front channel. for the front brakes. Is this a hazard? from what I read thats backwards and the big reservoir is to compensate for pad wear. other than that, is it dangerous? can i just keep an eye on the level for a few hundred miles? are the pressures the same? it took me waaaay too long to bleed the brakes and im tired of all the fiberglass in my arm from the header wrap...and my soft accountant hands are sore.

 

[ntsqd]

The problem is that now the proportioning valve is acting on the front brakes. Assuming that distribution block has one. It should be operating on the rear brakes. So yes, it's a hazard. I would not drive it until corrected.

 

[slowcpa]

all it takes is for one guy to say its not safe, whats another 3 hours bleeding brakes and slapping duct tape on my arm to get rid of the fiberglass fibers....good times. thank you.

 

[bigmuddy]

Yeah but what you don't know is this guy who responded really knows his shit when it comes to brakes...

 

[DirtDonk]

While some manufacturers did run front reservoir to front and rear reservoir to rear, most, including all older Fords, had the rear to the front and the front to the rear.
The position wasn’t as important as the reason for the position. The larger reservoir has more backup fluid, which is what disc brakes need. As the pads wear, the pistons extend and more fluid is taken up in that space.
This type of master was used even on drum brake vehicles as well, but it was during the beginning of the disc break era. So whether it was a disc or drum vehicle, they were prepared.

From your description, there is actually a proportioning/combination valve that is in use.
The original 74 system would’ve had a distribution block, or what we call a “H block“ but it actually has some other kind of official name that I can’t remember.
All that old one did was turn on a warning light on the dash in case there was a leak.
An actual combination valve has multiple functions. That rubber boot over a little nipple (that often leaks), is for the front brakes only. It causes a slight delay in the application of the front disc brakes, letting the rear drums apply first or at least catch up.
The rear system is where the proportioning aspect comes into play, and is for the rear only. As was earlier mentioned, you definitely don’t want that working against the front brakes.

For proper bleeding, that little rubber boot should be popped open, the little pin grabbed and held out while bleeding the front brakes.
Doesn’t matter for the rears, but it can be very important when bleeding the front system.

 

[ntsqd]

all it takes is for one guy to say its not safe, whats another 3 hours bleeding brakes and slapping duct tape on my arm to get rid of the fiberglass fibers....good times. thank you.

I suspected that my post wouldn't be welcome, even if you knew that's what needed to happen.

Usually the tube nuts at the master cylinder are different sizes so that they can't be accidentally swapped when replacing a bad one. I'd expect that they're all the same size at the distribution block/combo valve/whatever y'all want to call it. Might be possible to swap them there w/o needing to make new lines. If so, you may not need much bleeding to get the brakes clear of air. The faster that you can make the swap, the better. Know that the fluid will dribble out of the lines unless you can replace the lid with a tight fitting solid seal.

 

[slowcpa]

dirtdonk, you are correct. the prior owner installed disc brakes in the front. the below is the old and the new. it looked to me that the old was leaking from the little black rubber circle on the right side of the rear valve (new one doesnt have that). one guy to hold the pin, one to pump the brakes and one to work the bleeder screw. its going to be a family affair.

 

[slowcpa]

I suspected that my post wouldn't be welcome, even if you knew that's what needed to happen.

Usually the tube nuts at the master cylinder are different sizes so that they can't be accidentally swapped when replacing a bad one. I'd expect that they're all the same size at the distribution block/combo valve/whatever y'all want to call it. Might be possible to swap them there w/o needing to make new lines. If so, you may not need much bleeding to get the brakes clear of air. The faster that you can make the swap, the better. Know that the fluid will dribble out of the lines unless you can replace the lid with a tight fitting solid seal.

thanks for the advice. ill move fast

 

[Oldtimer]

The brake line to the rear needs to connect here. This is the proportioning section of the combination valve.