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67Mustang Al. 12-29-2010 04:28 AM

Bleeding Air from system with adj valve.
1 Attachment(s)
I am having a hard time trying to bleed the rear wheel disc brakes on the mustang and i am thinking that it has to do with the adjustable brake bias valve fitted into the rear brake line. The mustang has a new power assisted tandem master cylinder and i have no problem bleeding the front lines . The after market bias valve is adjustable and i am wondering if there is a technique required to bleed the air past the valve to the rear. ??? Maybe a pressure bleeding set up is required instead of the "pump the pedal" old style bleeding.
At this point i have dry rear bleed screws.
Any help appreciated.

cobalt327 12-29-2010 05:12 AM

The adjustable proportioning valve shouldn't cause the rear brakes to bleed any differently. That said, I'd adjust them to give as much rear brake as they'll allow, just for ease of bleeding them. Then return it to its normal position afterwards.

The dry bleeders leads me to think the master cylinder push rod might not be adjusted properly- there should be some clearance between the p-rod and the piston w/the brake pedal at rest. Be sure the return spring is doing its job and that the pedal cannot be lifted up further from its "at rest" position.

The master cylinder has to be bench bled. When you do that, it should be evident if the rear circuit is actually pumping fluid -and it's not frozen up.

Even if "new", master cylinders can be faulty, too, so don't discount that possibility.

cunningham 12-29-2010 09:51 AM

: Bleeding Air from system with adj valve. Reply to Thread
I know this may seem simple, but you stated that the front brakes bled fine. The correct procedure is to begin with the wheel farthest from the master cylinder, then work from there. I know that's pretty basic, so don't be offended.

cobalt327 12-29-2010 09:59 AM

That's true, but because it is a two reservoir master, unless the brakes are plumbed differently, the front brakes should be isolated from the rears, with no 'communication' between the two.

Still, the furthest front brake (right front) should be bled first, as should the right rear brake- but it doesn't matter if the rears or fronts are bled first in this case.

The largest reservoir is for the front discs. I'm now wondering if that master has the capacity to operate the rear discs?

technicaltom 12-29-2010 03:07 PM

Even if "new", master cylinders can be faulty, too, so don't discount that possibility.[/QUOTE]
After you verify that the mastercylinder is ok ,and preferably " bench bled". I would proceed as follows, first fit some suitable pipe on the front caliper bleed nipples and run the pipes into a clear container do the same at the rear,now open all the nipples just enough to allow fluid to flow,GET HELP to keep MC. full and gently pump the brake pedal , Make sure the pedal goes "fully" to the floor each stroke,Be generous and pump 1/2 pint /litre through , then close one nipple at at time,while you keep slowly pumping pedal.OK this procedure takes three people ,and lots of fluid , dont re- use the fluid that you have pumped into the containers beneath the car as it willl contain air bubbles.This is A VARIATION OF THE STANDARD bleeding procedure but it has worked for me on "Difficult" cars over the years.

67Mustang Al. 12-29-2010 04:31 PM

air lock
2 Attachment(s)
Thanks for your help guys. i have to question the master cylinder i think from what you have said. i had the two pistons out of the master cylinder as it sat in a box for twelve months while i stripped the car. I am wondering if i have fitted the first of the two pistons in backwards by mistake.??? I didnt think that was possible.
When i got the car all the new parts , disc conversion, power booster etc were fitted. The rear calipers were fitted to the incorrect side with the bleed screws on the bottom. The pedal was spongy as the air remained trapped in the top of the calipers. I have swapped them since.
What setup is needed to "bench bleed" a master cylinder. Is it a duplicate set of brake lines fitted to the output of the master cylinder while fastened to a bench and pumped??
thanks . This blue valve has a 2psi rating on it and an"OUT" marked on one end. I am assuming it is a residual pressure valve.??

67Mustang Al. 12-29-2010 05:01 PM


Originally Posted by cunningham
I know this may seem simple, but you stated that the front brakes bled fine. The correct procedure is to begin with the wheel farthest from the master cylinder, then work from there. I know that's pretty basic, so don't be offended.

Your right, and i would do that normally. This master has two lines out of it, one dedicated to front only and the other rear only. I will need patience with this one.
Thanks Al.

technicaltom 12-29-2010 05:09 PM

I am impressed , you are keen to solve this. :) Bench bleeding ,i usually gently clamp the master cylinder in the vice on the " bench" top up the revovoir with clean brake fluid,GET SOME RAGS so you dont make a mess, then push the rod in fully till fluid appears on all outlets. Refit the MC to your car and bleed the system . Best of luck/

67Mustang Al. 12-29-2010 06:26 PM


Originally Posted by technicaltom
I am impressed , you are keen to solve this. :) Bench bleeding ,i usually gently clamp the master cylinder in the vice on the " bench" top up the revovoir with clean brake fluid,GET SOME RAGS so you dont make a mess, then push the rod in fully till fluid appears on all outlets. Refit the MC to your car and bleed the system . Best of luck/

Thanks Tom. yes i have to solve this or i will not be able to keep on assembling the mustang. Had hoped to have this on the road before christmas. Just tried the adjustable bias valve fully open and then fully closed but still dry at the rear calipers. . Have to take out the master next i think.
Thanks Tom :thumbup:
Cheers Al.

cobalt327 12-29-2010 07:21 PM

The bench bleeding procedure calls for the outlet ports being plumbed back into the partially filled reservoir. Usually, plastic fittings w/the correct thread are supplied w/the master cylinder that have a nipple that you attach a piece of plastic tubing to. The tubing then goes back into the brake fluid in the reservoir.

Both front and rear ports are done at the same time.

If you look closely, you can see the lines leaving the outlet ports, looping back into the reservoir in the image below:

Another way to bench bleed the master cylinder that recently came to my attention when I bought a master cylinder for my '80 Malibu, is to completely block the outlets from the master w/plugs. So nothing can escape. Then the piston is depressed until no more bubbles issue from the ports located on the bottom of the master reservoir.

The master cylinder piston shouldn't be bottomed out during either bench bleeding procedure.

Whether or not this will also work on other master cylinders, I don't know.


Even though I have a MityVac, I find it too small, volume-wise, to be a very convenient way of bleeding brakes.

It's actually easy to make a pressure bleeder from a garden sprayer. Some use air pressure to push the fluid from the master cylinder reservoir through the system. This requires you to keep a close eye on the fluid level so it doesn't run out during the procedure.

Another way is to use a small volume sprayer and pour the fluid into it and pressurize the master cylinder reservoir w/fluid (not air). This means no chance of the fluid running dry- but does mean the sprayer pressure has to be watched and maintained and the master reservoir needs to be drained back to the correct level when done.

I have mixed feelings about saving the brake fluid once it's been poured into the sprayer tank, but I suppose it would be OK as long as the tank was spotless and dry. But not if any old, discolored fluid is allowed to contaminate it- like by siphoning the excess fluid from the master when done bleeding; I would put that into a separate container to be tossed.

I use a rather large (2 gallon) sprayer. This gives a good amount of "run time" before the tank needs repressurized. But instead of pouring the fluid out into the sprayer's tank, I leave the brake fluid in the quart container it came in. I remove the cap, insert the suction hose of the sprayer and that's that. Towels are used all around the quart container to support it and keep it from tipping over. ;) And, for cripes sake, if you make a bleeder like this- keep the wand- just unscrew the tip, so you can turn it off and on at will. A couple of the tutorials show the whole wand cut off and I believe that's a mistake.

There are several tute's online, here are a few:

67Mustang Al. 12-29-2010 11:03 PM

Master Cylinder
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Hi cb 327
Thanks for taking the time to help me out here. Those ideas and web sites have given me some homework. :) I have just removed the master cylinder and bench mounted it. Pushed a steel rod down the end to simulate the booster push rod and only saw fluid coming out of the reservoir for the front brakes. Nothing at all coming out the rear port for the rear brake line. So its either air loccked or something wrong with the second piston and its seals.
I removed the circlip and primary and secondary pistons from the bore. Although the cups seals look okay there was seeping brake fluid out the end past the last cup seal.
I will try and rig the cylinder as per your daigram or net and see what result is get. Thanks 327
Al from downunda.

OLNOLAN 12-29-2010 11:15 PM

Bad Rear M/c Function
Hey Al, I have run into more than one master cylinder that pumped on the front and not on the rear. Sounds like its time for a new one. Is the combo valve block for a disc/disc setup? I didn't think you needed a residual pressure check valve unless the master cylinder was lower than the calipers. I noticed you have one in the photos. olnolan

1971BB427 12-29-2010 11:39 PM

Before you replace anything you need to either pressure bleed, or put a vacuum pump on the output side of your master. I had this identical problem with the brand new master I put on my Austin gasser which came from a Subaru Legacy. I got nothing from the rear outlets, at all when in the car, or bench bleeding.
I called a friend who works as frontend tech at the local Les Schwab and he told me they can be air locked, and need to be evacuated by pressure or vacuum.
I put a vacuum pump on the master until I got fluid out of both ports, then connected the lines again, and pulled the wheel bleeds to pull fluid to each wheel. After doing this the master worked perfectly and I got fluid out each bleed when I did the std. type bleeding by pumping the pedal.
I've never had this ever happen on a master before, and really was skeptical it would work, but I'm a believer now!
My Subaru master has twin ports for the front and rear , so it will allow individual lines to each wheel. I plugged one rear port, as I didn't need two lines going to the rear.

67Mustang Al. 12-30-2010 05:26 AM

Pesky master cylinder.
Hi 427 .
As this master is new and you had the same problem i will follow your advice and have a crack at getting it to flow out of the rear port by figuring out how to apply pressure to rid the casting of the air lock. cobalt 327 had some websites to have a look at for bench bleeding master cylinders.
When i pulled the pair of pistons out today the cup seals looked okay and i was puzzled how there could be an airlock in the casting, but when i depressed the primary piston , like you i only had fluid out of the front port and nothing from the rear.
Olnolan also suggested the residual valve may not be needed for this 4 wheel disc setup ( i think it will hold 2 psi on the rear caliper pistons) Would this cause the rear pads to drag slightly ( something else to figure out.)
Back to the main issue of getting this m. cyl. to cooperate. I dont think the residual valve is the problem.
It the subaru tech had the answer then i will try that tomorrow.
I appreciate the help from the experienced guys here . Far better than being an aussie deer with no eyes, no ideer. :thumbup:
Back to garage to have another go.
Cheers and thanks.

OLNOLAN 12-30-2010 06:36 AM

Brake Fluid Incompatability
Hey Al, Just to throw out food for thought, make sure not to mix different brake fluids when bleeding, setup, etc. DOT 3/4 mixed with DOT5 can cause the seals to fail. They go from nice firm rubber to swelled up pieces of mushy crap. This is something the brake fluid companies don't mention. I think its either one or the other with no cross contamination. You may want to read up on this too.olnolan

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