Here is a good description of the brake system function including the pressure differential switch/isolation valve.

http://www.classicperform.com/ShopTalk/

There is a tool to use when bleeding or servicing the brake system to keep the pressure switch/isolation valve centered. Before the hydraulic system is opened, the pressure differential switch connector on the combination valve is removed. In its place, the tool is threaded into the port which has a small pin on it to hold the isolation valve in the centered position.

http://www.npdlink.com/store/products/tbird_brake_pressure_differential_valve_switch_lock_tool-178818-6444.html

57 Black Tbird 312/auto

If the brake system has been opened not using the lock out tool and the isolation valve has gone off center and either the front or rear brakes won't bleed, here is the way to recenter the pressure differential valve. Make sure all four bleeders are closed. Have a helper sit behind the wheel with his foot on the brake but not pushing it down. Open a bleeder on the system that system that will bleed (front or rear) while having the helper very slowly press the brake pedal down until a pop is heard or felt in the brake pedal travel and hold it in that position without moving it. Close the bleeder. The isolation/differential valve is centered and the warning light should be out. I had to do this as an apprentice a few times before I listened to the journeyman explain, once again, how and why to use the lockout tool. I still have mine from 1979.

57 Black Tbird 312/auto

So the brake warning switch (isolation valve) does most of the work protecting you from brake failure, that's what was told to me and is now confirmed by the link provided by Mark.

I wonder what would happen if you installed an isolation valve (see I learned a new term today) on a single master, split the brake system front to rear and pipe it to the outlets then connect the 2 inlets to a stock single master cylinder. Would that protect from any failure on the down stream side of the valve?

Sure you could still have a master cylinder failure but I have never seen one fail without warning such as sinking pedal or leaking at the pushrod

Seems that anything short of a full-on disc conversion is a 50/ 50 proposition, at best and would come at the expense of losing my power assist. I'm not really in a position to let go of all that $ to do a conversion right now, either. I think I'll just have my booster rebuilt, and buy a big anchor w/ 50 ft. of good heavy chain, in the meantime 'til I can scrape up the coin to do it right. I am still going to send my wheel cylinders out to be sleeved w/ .304 this week however. /Duck

BOO- YA!!!

Hey there, Duck cruzin' with a heavy foot again? Wear the boots with the brake linings...........

Stargazer had a note on here (I think?) about using a different divorced brake booster. Apparently, we are (as usual) being victimized by vendors on account of wanting the "original" parts - when another nearly duplicate part is available for less (and looks just like it).


 
 Steve Metzger       Tucson, Arizona

I think this is the tread Steve is referring to... http://forums.y-blocksforever.com/Topic84997.aspx#bm86383

This doesn't answer the dual circuit/single booster question for drums. Boosted front-only seems to work well for disk-drum setups, but I don't think I would want to boost only the front of an all drum setup.

Steve- no, this thing's an expensive paperweight until either I can 1) get the brakes sorted or 2) the return of nice weather (whichever comes last) I also smell subterfuge inasmuch as vendors "whitebox" parts. Nothing new there though, I suppose. Seems if anyone had ever successfully accomplished what I had hoped to do, I'd have found a trace of it on the interweb, by now. After reading about M/C's & distribution valves/ switches not working as intended, I'll be more conservative in my thinking for safety's sake. Thanks for the head's up. /Duck

BOO- YA!!!

There appears to be a lot of misinformation and misunderstanding of how a dual master cylinder works. There are two separate braking systems, they never share fluid. The isolation valve is just that, to stop leakage from the system that has the leak. It does nothing for the fluid in the system that is working. I find it odd that the link Mark provided doesn't mention anything about the internal safety devices in the M/C.

Here's is an interactive explanation on how the dual M/C works to prevent total loss of your brakes. As said before if your brakes are not set up correctly to begin with or your travel is insufficient then the safety factor is not going to work.

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/master-brake.htm

My own brake system on my '55 is out of a '72 Granada, M/C, power booster, disk brakes and rear brakes. I don't even have the "isolation valve/warning light installed, nor is it needed. It is just an another safety item in ADDITION TO the dual M/C.

Am I a brake specialist. No, but I have done quite a bit of research on brakes.



"The Master Cylinder"

Enjoying life at the beach in SOCAL

The link I posted deals with the design and operation of a typical combination valve. The dual circuit master cylinders are two separate braking systems in one master cylinder body. The weak point of the design is pedal travel related to incorrect rear drum brake adjustment and/or worn linings. If the front brakes are lost do to a hydraulic failure the pedal needs to travel enough to move the front master cylinder piston to make contact with the rear brake piston engaging the rear brakes. If the rear brakes are not adjusted correctly or a combination of worn linings and poor adjustment, the pedal may hit the floor/carpet before making adequate movement to engage the rear master cylinder piston with enough pressure to initiate adequate braking. The brake warning light was designed for those less than "attentive" drivers who may continue driving with only one system working, particularly if the rear brake are inoperative due hydraulic failure. The isolation valve closes off the master cylinder from the failing system minimizing fluid loss.

57 Black Tbird 312/auto

Haha, Mark, You said it all right there... If your braking system is borderline due to lack of maintenance or you installed it improperly and didn't check it in the first place, how can it be blamed on the dual master cylinder/dual brake system when it doesn't work properly.

One thing I learned in class (way long ago) is you can't design for "Double Jeopardy". For one example, lack of maintenance and failure of a component.

Bottom line I feel when set up properly and maintained a dual braking system is way safer than a single M?C system.

"The Master Cylinder"

Enjoying life at the beach in SOCAL