Help Bleeding/Resetting Pressure Differential

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Ive got a couple of questions about bleeding the lines, resetting the pressure differential switch/valve, and finding this particular 'brake bleeding tool'. But first...

My 67 Plymouth Fury VIP has factory drums front and rear. The entire car has 14,410 original miles. The brake light has been lit up on the dash since I got her. The brakes performed badly to say the least.. pedal spongy or all the way to the floor. So far I have done the following:
- installed 4 new wheel/slave cylinders.
- Had the factory brake booster rebuilt.. beautiful job by Mr. Booster Dewey at Power Brake Booster Exchange in Portland. Included in the rebuild was a re-anodizing & zinc plating of booster housing and master cylinder lid to factory finish.
- Had the factory master cylinder over bored and stainless sleeved to factory 1" diameter.. beautiful master machine work by Brad over at Brake & Equipment Warehouse in Minneapolis. They also did an exceptional job powder coating it to its original factory gray finish.. all for $120
- Then I rebuilt the master using a complete Mopar NOS rebuild kit including new pistons, springs, brass seats, valves and lid gasket..all rubber was in perfect shape.. $30 for kit on ebay.
- I bench bled the master.
-Reinstalled booster and master
-With the help of a friend, we proceeded to bleed the system.. This was about a year ago and I don't remember which, but the flow/bleed was only trickling out at either the front or rear, I think it was the rear. We bled each wheel in the correct sequence until we got a hard pedal, refilling the cylinder as necessary.
But, the pedal became very spongy again on the first test drive around the block, and the brake warning light remained lit. We bled them again. Same result. My friend then suggested that I needed to change the brake distribution valve (which some erroneously call a proportioning valve.. mine is a distribution block). At the time I knew nothing about how the brake pressure differential valve trips/shorts out the brake warning switch/sensor- turning the dash light on. Sooo, instead of figuring out how to reset the switch, I went hunting for a new distribution block. For my year and model there were NONE being manufactured.. so I searched & searched and finally found an NOS brass block still in the box online at AMS Obsolete... for a mere $289.50! They also sold me an NOS Tee-block for the rear lines. I then purchased new factory correct brake hoses.
A year has gone by and I'm back at it.. getting her up to speed once and for all (and not just brake stuff).
I still have these parts unused in their boxes.. which leads me to some questions for the brilliant minds of FCBO members..
1. How do I reset the pressure differential valve/move it back over to center? I've heard a couple of methods mentioned around youtube.
Any experience with successful results?

2. Once the valve is reset/centered, I'll apparently need a doo-hickey 'brake bleeding tool' that screws in to replace the brake warning switch, which will lock the pressure differential in place while I bleed the lines. Once bled, then this 'tool' is removed and the warning switch replaced. I'm confident this will work.. So where do I get this 'screw-plug-lock-tool' that will fit my car?

And one last thing- will taking it out after bleeding to put the switch back in allow air to enter the system?

Much obliged in advance!
 
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The pressure differential switch you are having problems with may call for replacement in the FSM, once it has tripped. That is not the usual way this is resolved however. To reset the switch, you may be able to get it to move by forcing hard pressure on the pedal and opening a line on the good half or the system (the half that bleeds nicely). Otherwise you may have to take the line(s) off to gently push the switch back to center. be very careful not to create any burrs which could block it from free movement.

I can't say I have ever seen the bleeder tool in use. But I have been aware of the existence of such items... even when the tool was in the parts room of the dealer, I doubt it got used much.

Once the switch has been reset, which will turn the light off. You should bleed the system with gravity bleeding or vacuum bleeding to get most of the air from the system. Pressure bleeding is possible, but the equipment is expensive and the pressure would need to be very small to minimize the chance of tripping the switch again.

Once you have somewhat reestablished a firm pedal while keeping the light off, you can do a 2 man bleed. Light pedal pressure is all you need during this bleed, too much force may cause the switch to trip again.

For those who already know, the rest is just bleeding technique.

HOW TO BLEED:

Gravity bleeding, is pretty easy... once the switch has been reset, open one or more bleeders (I recommend one at a time) and take the cover off the master cylinder... keep it full and place a drain pan under the position you're bleeding. For less mess and cleanup, use a piece of hose and an old clear bottle to catch the fluid that comes out.

Vacuum bleeding requires a tool... cheap tool store versions are under $100. they would be close to the gravity bleeding directions (need to leave the cap off) and require an air compressor to operate. The main advantage is time... the amount of time the fluid is open to air affects it's saturation with moisture (DOT 3, 4 and 5.1 absorb water).

Pressure bleeding tools are very expensive. This is the best method, but the equipment required costs so much even small shops tend to not have it. Be aware the cheap bug sprayer kits that you may find are not a very good option... the leftover fluid must be thrown away after use, as it has had direct contact with air. The adapters for using these kits are also pricey.

2 man bleeding is common, and screwed up a lot... even by so called pros. You never want to allow the pedal to push all the way to the floor. Often master cylinders are not finish machined for the piston cup seals to travel that far. My favorite trick is to lay a 2x4 block under the pedal flat, so that it blocks that final travel of the pedal. The assistant who is pushing the pedal often believes they need to push hard... they don't. If they push too hard you may trip the pressure differential switch. The person who controls the bleeder screw is in charge of this operation, they tell the person at the pedal to push "down", then open the screw enough for air/fluid to come out and ideally close it again before it has relieved all pressure from the circuit. Once the bleeder is closed, they allow the pedal pumper to let off, "up". It will usually take many pumps to bleed air out this way... the system works off of pressure and doesn't push a very large volume. Bleed all wheels and close all bleeders before pushing hard on the pedal to check the system.

2 man bleeds are also often used as a secondary bleed after a different method was used... they make it easy to assure no air was left behind. Watch the bleeder screw for air bubbles while doing this. All of these methods can be done with the hose and bottle to minimize the mess, vacuum bleeding already has the hose and catch container.
 
All of what Cantflip said x2.

I've had those differential valves trip before and they've always come back to center. It may be worth buying a used one from Murray Park if you can't get yours to work.

I use a MityVac vacuum pump myself. You can find it other places, but it's $40 from a Harbor Freight store (find a coupon and get it cheaper). It's handy for a lot of things.

Mityvac Vacuum Pump

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The pressure differential switch you are having problems with may call for replacement in the FSM, once it has tripped. That is not the usual way this is resolved however. To reset the switch, you may be able to get it to move by forcing hard pressure on the pedal and opening a line on the good half or the system (the half that bleeds nicely). Otherwise you may have to take the line(s) off to gently push the switch back to center. be very careful not to create any burrs which could block it from free movement.

I can't say I have ever seen the bleeder tool in use. But I have been aware of the existence of such items... even when the tool was in the parts room of the dealer, I doubt it got used much.

Once the switch has been reset, which will turn the light off. You should bleed the system with gravity bleeding or vacuum bleeding to get most of the air from the system. Pressure bleeding is possible, but the equipment is expensive and the pressure would need to be very small to minimize the chance of tripping the switch again.

Once you have somewhat reestablished a firm pedal while keeping the light off, you can do a 2 man bleed. Light pedal pressure is all you need during this bleed, too much force may cause the switch to trip again.

For those who already know, the rest is just bleeding technique.

HOW TO BLEED:

Gravity bleeding, is pretty easy... once the switch has been reset, open one or more bleeders (I recommend one at a time) and take the cover off the master cylinder... keep it full and place a drain pan under the position you're bleeding. For less mess and cleanup, use a piece of hose and an old clear bottle to catch the fluid that comes out.

Vacuum bleeding requires a tool... cheap tool store versions are under $100. they would be close to the gravity bleeding directions (need to leave the cap off) and require an air compressor to operate. The main advantage is time... the amount of time the fluid is open to air affects it's saturation with moisture (DOT 3, 4 and 5.1 absorb water).

Pressure bleeding tools are very expensive. This is the best method, but the equipment required costs so much even small shops tend to not have it. Be aware the cheap bug sprayer kits that you may find are not a very good option... the leftover fluid must be thrown away after use, as it has had direct contact with air. The adapters for using these kits are also pricey.

2 man bleeding is common, and screwed up a lot... even by so called pros. You never want to allow the pedal to push all the way to the floor. Often master cylinders are not finish machined for the piston cup seals to travel that far. My favorite trick is to lay a 2x4 block under the pedal flat, so that it blocks that final travel of the pedal. The assistant who is pushing the pedal often believes they need to push hard... they don't. If they push too hard you may trip the pressure differential switch. The person who controls the bleeder screw is in charge of this operation, they tell the person at the pedal to push "down", then open the screw enough for air/fluid to come out and ideally close it again before it has relieved all pressure from the circuit. Once the bleeder is closed, they allow the pedal pumper to let off, "up". It will usually take many pumps to bleed air out this way... the system works off of pressure and doesn't push a very large volume. Bleed all wheels and close all bleeders before pushing hard on the pedal to check the system.

2 man bleeds are also often used as a secondary bleed after a different method was used... they make it easy to assure no air was left behind. Watch the bleeder screw for air bubbles while doing this. All of these methods can be done with the hose and bottle to minimize the mess, vacuum bleeding already has the hose and catch container.
Excellent tips and tutorial- Thank you for reading and responding! I'll also be consulting my handy service manual.
As for the switch, I'll most likely remove the line(s) as you suggest, and use a thin pick/tool to carefully slide/push it back over to center. I'm so glad I won't need another 'bleed tool', just a light foot on the break pedal.
When my friend and I initially did the 2 man method we did not place a block under the pedal.. he was at the bleeder valves, and under his instruction I applied very hard pressure all the way to the floor several times... Are you saying that this may have shredded or damaged the new piston cup seals? The brake fluid was clean and the cylinder was just freshly machined/sleeved before installing the pistons, whether it was smooth machining all the way to the bores' end I didn't notice.. could have been!

Also, I'm more likely to do a gravity bleed by opening all the bleeders at once. With the extra time for air exposure being a concern with this method, wouldn't the follow up with the two man method eliminate that contamination?
 
All of what Cantflip said x2.

I've had those differential valves trip before and they've always come back to center. It may be worth buying a used one from Murray Park if you can't get yours to work.

I use a MityVac vacuum pump myself. You can find it other places, but it's $40 from a Harbor Freight store (find a coupon and get it cheaper). It's handy for a lot of things.

Mityvac Vacuum Pump

View attachment 131600
Hey Big John, thanks for the reply! When you say they've always come back to center, how do you mean?
I have an NOS block in a box.. but based on what I'm hearing most times it can be corrected without replacement.

I'll definitely look into the mityvac too.
 
Hey Big John, thanks for the reply! When you say they've always come back to center, how do you mean?
I have an NOS block in a box.. but based on what I'm hearing most times it can be corrected without replacement.

I'll definitely look into the mityvac too.
If you look at a cross section of the valve, you can see it's a pretty simple piston that is spring loaded to center. When the pressure on one side is enough to overcome the spring and push it so it contacts the switch and lights the light. It should return to center once the pressure is equal, but I'm sure there are factors that will make it stick and not return.

Brake Switch.jpg
 
I've never had one apart, but I'll bet you could take it apart, clean and put new O rings in it.
 
If you look at a cross section of the valve, you can see it's a pretty simple piston that is spring loaded to center. When the pressure on one side is enough to overcome the spring and push it so it contacts the switch and lights the light. It should return to center once the pressure is equal, but I'm sure there are factors that will make it stick and not return.

View attachment 131607
Thanks John, good diagram.
I'm eager to move it back to center by hand (which i did not do the first time I bled the lines) and then do a full bleed. Isn't it virtually impossible to bleed the lines properly and get equal pressure while the switch is tripped? I'm looking forward to solving this very soon, and having a hard.. even touchy brake pedal. Thanks for the great information.
 
I've never had one apart, but I'll bet you could take it apart, clean and put new O rings in it.
That way I could shelf my NOS valve for a rainy day, another 50 years from now.
 
And now suddenly they're everywhere for half the price I paid.. here for $165: NOS Mopar Brake Tee 1966 Thru 1969 Plymouth Dodge Chrysler Imperial - Hiltop Auto Parts.

Clearly I got bamboozled by that AMS Obsolete.
AMS is funny... They still hold on to the reputation of the former owner, Frank Mitchell, who could be a very difficult person to deal with. He had the parts though... Some were very expensive, but he had them. Frank sold the business and then passed away about 3 years ago.

I've bought a few things from them over the years, mostly on Ebay. They haven't let me down at all. The parts were as promised and shipping was prompt. The prices were sometimes a little high, but the last NOS piece I bought from them wasn't that much more than the used one I bought from Murray Park.
 
Excellent tips and tutorial- Thank you for reading and responding! I'll also be consulting my handy service manual.
As for the switch, I'll most likely remove the line(s) as you suggest, and use a thin pick/tool to carefully slide/push it back over to center. I'm so glad I won't need another 'bleed tool', just a light foot on the break pedal.
When my friend and I initially did the 2 man method we did not place a block under the pedal.. he was at the bleeder valves, and under his instruction I applied very hard pressure all the way to the floor several times... Are you saying that this may have shredded or damaged the new piston cup seals? The brake fluid was clean and the cylinder was just freshly machined/sleeved before installing the pistons, whether it was smooth machining all the way to the bores' end I didn't notice.. could have been!

Also, I'm more likely to do a gravity bleed by opening all the bleeders at once. With the extra time for air exposure being a concern with this method, wouldn't the follow up with the two man method eliminate that contamination?
Being sleeved... I would bet you're OK with the master cylinder. Careful on the gravity bleed, there is a likelihood that some circuits will get there first and 4 wheels wouldn't take long to empty the reservoir... watch that closely for all DIY methods.

One more that may be perfect for you... it always slips my mind because I never used it. they sell bleeder screws with check valves for DIY brake bleeding. $12 a pair last I saw... might make life much simpler.
 
Thanks again everyone. I'm off and running.. and soon, stopping.
 
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