Brake Guru's...Need a Bit Of Advice.

Your problem is your master cylinder. The stock master can't handle the extra volume needed for the 4 wheel disks. You'll need to find a larger bore master that will fit.

the problem you have is that drum brakes rely pretty much on just pressure when you apply them, where as disc brakes require volume as well as pressure. you can use the master cylinder from a lincoln towncar with 4 wheel disc brakes, as it will bolt to your booster.

I have a 1984 Mustang SVO MC with my 4-wheel disc. Works well, all aluminum and was like $25.00

I don't have power brakes, though..

gram, did you remove the residual valve from the stock master?

it should be in the port for the rear brakes, i can't remember right off the top of my head how to get it out but i'm pretty sure darkbuddha and a few others have done it, might try pm'ing db if someone doesn't jump in here in a bit.

Got your PM and I'll offer what I can. I have yet to finish my rear disc install, but I've been doing the reasearch and emailing some folks "in the know". You're right about finding a master that will fit right... it's a tight fit. I tried a couple different ones and found that most won't clear the shocktower. My decision has been to stick with the stock disc/drum master as well. The biggest problem is the residual valve in the rear port which has to be removed for it to work, otherwise the rear discs may drag. To remove the residual valve should be as easy as removing the line to the rear brakes, screw a decent length sheet metal (coarse thread) screw into the port, and pull... the residual valve should come right out. Oh, and I have heard the Versailles/Granada 4 wheel disc master is smaller than most others, so you might check that option if you haven't already.

Now, all of that said, I'm not sure that is necessarily going to solve your problem. The kind of problem you're describing is a tough one to diagnose. You've bled the master, bled the lines, etc... the usual culprits. You're not losing fluid, or you'd feel the pedal sink more. I'm also going to assume you're using the stock booster, master, pushrod, and the original power disc pedal which means all those components should be fine. So, what is left???

You mentioned a 4 wheel disc specific prop valve/distribution block... was it new or used? Are you using it strictly for the brake warning light??? I wonder if it might be your culprit. In the August 2005 issue of Car Craft, a Bendix rep chimes in on some brake bleeding issues of one of the readers. The reader couldn't get fluid to his rear brakes at all because the proportioning valve. Apparently, many proportioning valves are designed to close the circuit if there is a loss of pressure, thus preventing a complete loss of fluid, like during a line leak or if you've taken the system apart. The valve has to be reset in order to function correctly and allow full fluid pressure to the circuit again.

As a side note about the proportioning valve: you may not need it at all. It may be possible that the front brakes can be plumbed straight to the master. The rears can probably do the same, except you'll need an adjustable proportioning valve to balance the system.

Ultimately, there is one last solution to these kinds of problems that has worked most often for me... take it all apart again and put it back together again. It almost always works then.

I hope this helps a bit.

dolfan87 said:

I wonder if having the 13" Baer's on the front makes it even worse?

Click to expand...

I just re-read this and something struck me... one of the important things about brake systems is the master cylinder bore, which dictates how much fluid will be displaced when the pedal is pressed. Now, depending on the Baer system you have, I wonder if the stock master has an adequate bore diameter to move the amount of fluid necessary to move the pistons of the front calipers. Remember, disc brakes are dependent on volume rather than pressure, so it seems logical that too small a bore would require more travel to push the needed amount of fluid.

You might call/email Baer about the recommended bore for your setup... Anyone know the bore specs of the stock disc/drum master???

hmmm........that one slipped my mind and like DB says that is a pretty big item to look at. IIRC the stock disc/drum master on the 69/70 mach's was only 7/8" or so. you probably need something with at least an inch, but probably more like 1-1/8" or 1-1/4" bore. i think the 67 mustang power disc master cylinder had either a 1" or 1-1/8" bore because it used the 4 piston kelsey hayes calipers up front. i'm sure it would work well in this application as well. note it may be kind of difficult to find one since it was a 67 only item. i'll see if i can find a part number and the specs for ya. be back in a bit.

well i looked at some of the parts places and they all show a 1" bore from 67-70. if yours has a 1" bore, even the 4 wheel disc piece from a granada is a 1" bore. the only thing i could find with a 1-1/4" bore was a 70 f-250 3/4 ton pickup. bendix part no 11779 for a new one and R11779 for a rebuilt. the design looks the same but i can't tell for sure if they are the same size or not. it looks like it would be worth looking into though.

as for the baer unit you might be able to grind the end of the master down a bit to make it fit. i've heard or people doing this with some MC's but i'm not sure how much grinding it would take.

I agree that disk brakes need a larger bore master cylinder from what I've always read. So, I suspect you have a problem with the bore size for your rear disks as well. I'm running Granada front disks and a Versailles rear, which is, however, manual at the moment. I used the master that Mustang's Plus sells for this application, and it seems to work quite well on a '65. I can't speak to shock tower clearance since my setup is non-power, but I can say that I think you have more clearance to the towers than I do. Have you looked at Mustang Steve's booster? It works on a dual bore master in a '65, but I can't remember whether he claims it will work on a '70.

Another option would be a later model, 4 wheel disk specific, plastic reservoir type master. There are some threads you could search on this topic that I have seen. I obviously have no first-hand info on these, but they do appear smaller and should be much closer in the ballpark in terms of bore size. I will end with a caveat, however; I have no idea as to what bore size the big Baer brakes would be happy with, although I must admit that I wish I had that problem.

I just saw bnickel's post after I had submitted my reply. I think he's right on about the bore size. Without searching for the thread about the late model masters, however, I seem to recall that in the thread I referred to there was some mention of a 1 1/8 bore size. I can't recall whether it was an SVO master or a SN 95 master or what. If you have any trouble locating the thread, let me know and I'll give it a shot. My bed is calling me at the moment.

O.K., one final note before bedtime (or past bedtime; damn this hobby/addiction!!!!). Check out the following link, which may offer you a viable option for a much more stock if that's important to you:
http://www.cobraautomotive.com/new products.htmook