Bleeding Master Cylinder

[mark69sportsroof 31]

OK update.  Took the mc off and had a local mechanic bench bled it.  Put it back on the brake booster and tighten down and attached the two brake lines.  Then I realized the engine bay electrical were between the mc and exhaust manifold.  Took the 2 mounting bolts off the mc and pulled it forward and up to slide the wiring

between the mc and shock tower.  Tighten the 2 mounting bolts.  I didn't disconnect the 2 brake lines.

 

Checked all lines for leaks and no leaks.  I'm not sure if the proportional valve is centered (I figured probably not) so I go the PS front brake and opened the bleeder

and stomp on the brake pedal one time and retighten the bleeder screw.  I also did the LR brakes the same way trying to re center the valve if it was off.

 

Pedal pressure feels normal when the engine is off.  So I start the engine and once again I lose pressure down to the floor.  I have just enough to stop it going 10 mph and pumping the pedal.

 

I drive 1/4 mile to get gas and notice I have a puddle under the MC area.  Looking for the source I discover the mc to the brake boost is leaking underneath the

mounting bolts.  I take the mc off totally to clean it.  Re mount the mc and re secure brake lines.  No leaks.  I bleed all brakes again in the sequence of RR, LR, FR and FL.   The brake pedal does to the floor as before. Check MC and no leaks anywhere.

 

Frustration level is rising.  It has been 4 months fighting the brakes

 

Thanks Mark

[Rsmach1 71]

If it was leaking between the m/c and booster the rear seal in the m/c is bad. The fix........return the m/c and get a new one.

 

Edit, once the m/c leaks from the rear it won't stop, you're not seeing it right away after cleaning because it's leaking into the booster. Unfortunately this just happens on occasion, and is most likely why you've had such a hard time With your brakes. Also when you bench bleed a m/c never bottom the piston out in the bore as this can damage the rear seal.

[Brian Conway 264]

http://www.mustangsteve.com/fyi_brakepedals/msFAQbrakepedals.html

 

.Well Mark ​sound like you've about had it.  It isn't really this hard so if your willing; how much of your brake assembly is Original Equipment ?  You have said this car is/was a project so start with the pedal assembly and verify hanger, pedals, brake rod, master cylinder, brake booster and the pressure differential/proportioning valve.  Go to the Mustang Steve site and verify that you have the correct pedal.  Little tough to get in there to measure the brake rod post but I think you need to do it.  I already posted the NAPA part number for the MC I am using   Which booster Midland or Bendix ?  Should be Bendix 6945.  As mentioned bleeding the MC will take short strokes, don't bottom out, this will take forever.  You must eliminate even the tiniest bubbles and there will be many.  Brian

​

[LoneWolf2U 136]

id not see anywhere that you have checked the boosters push rod to the master cyl bore depth.

this will cause the problem you are having with poor braking.

[RPM 1,190]

so I go the PS front brake and opened the bleeder

and stomp on the brake pedal one time and retighten the bleeder screw.  I also did the LR brakes the same way 

 

 

Thanks Mark

 

Are you bleeding them by yourself, or do you have a helper?

[Ridge Runner 1,112]

It sounds like you are opening the bleeder then pumping the pedal then going out and tightening the bleeder ? That will not work .You need someone to tighten the bleeder as you hold the pedal down or it will suck air back into the system when you release the pedal . You can open the bleeder, and with a vacuum line on the bleeder and into a bottle of brake fluid ,pump the pedal slowly and it will force the air out and suck break fluid back in .Tighten the bleeder while the vacuum hose is still in the bottle of fluid. 

[mark69sportsroof 31]

WOW, thanks for all the info. Let me try to answer all the questions.

 

RMmach1 - I took off the MC again last night and cleaned the outside of it, remounted and just pumped the brakes and no leaks.

Just checked it again and it was leaking again in the same spot. You are right - back to napa for a new mc.

 

Brian - I have all new brake system from the local mustang store. I have a rebuilt Bendix 6945. I have the correct pedal setup,

and the 68-69 reproduction (original style) proportional valve.

 

Mach1Rider - the guy you rebuilt the brake booster and had the mc and measured the mc pushrod so it would be the correct length.

Do I need to have it checked again that a have a new mc?

 

RPM - I have a helper. Mother in law does the brake pedal, I do the tires

 

Ridge Runner - I do 90% of what you said. I hook a rubber hose to the bleeder then loose it. I have the other person pump the brake pedal 3x slowly and on the last one hold the pedal to the floor. Then I tighten the bleeder. I do this 3x on each wheel.

The difference is the hose in the bottle is not in the brake fluid to suck it back in. I must be sucking air back in the system.

 

Sounds like I need the another new mc. Then bench bleed it. Bleed brakes with the additional step of the hose in the bottle submerged in the fluid.

 

Thanks Mark

[Brian Conway 264]

Close the bleeder valve after all the air bubbles have been pushed out and while the brake fluid is still flowing.  You STOP the flow at the wheel cylinder while gentile pressure is still being applied at the peddle.  Brian

[Rsmach1 71]

I think the new m/c will solve your issue, just remember don't bottom out the piston when bench bleeding it. Short 1" or so strokes.

[DRASTiK 22]

I hook a rubber hose to the bleeder then loose it.  I have the other person pump the brake pedal 3x slowly and on the last one hold the pedal to the floor. Then I tighten the bleeder.

 

This may just be a typo, but to clarify - do not loosen the bleeder BEFORE having someone pump the pedal 3 times. 

 

Have them pump the pedal 3 times and then hold pressure on the pedal (bleeder is still closed).  THEN, loosen/open the bleeder to let air/fluid out while pressure is still being applied to the pedal.  Then re-tighten/close the bleeder while the pedal is still being held down.  Repeat as needed.

 

Hope the new master cylinder fixes your issue. 

Dave

[mark69sportsroof 31]

Another update - received a new (not rebuilt) mc from NAPA. I benched bled it using a tube kit and by the manufacturer instructions. Attached it the brake boost, attached the lines.

 

Had my 83 year old mother in law doing the brake pedal. I did the correct sequence and had her pump it 4-5x and hold the pedal. Iopened the bleeder screw, pedal went to the floor. She held it there and I closed the bleeder screw. Then she released the pedalDid each brake 4-5x.

 

Brake pedal seemed normal. I started the car and had about 50% at best. Drove it a few miles and checked for leaks. Once again

it is slowly leaking on the bottom side of the mc where it attaches to the brake boost. Unbelieveable.

 

I am picking up my 3rd mc from NAPA. The first mc didn't leak there but the last 2 from NAPA has. What should I check for on the mc or the brake boost to figure out why it's leaking?

 

Thanks for you advice

 

Mark

[Brian Conway 264]

Close the bleeder valve after all the air bubbles have been pushed out and while the brake fluid is still flowing.  You STOP the flow at the wheel cylinder while gentile pressure is still being applied at the peddle.  Brian

 

The brake pedal should not go to the floor.  GENTLE pressure is all that is needed.  What MC, part number, from NAPA are you having all this trouble with ?  Brian

[mark69sportsroof 31]

I was stopping the fluid flow while it was still coming out of the bleeder screw. Got the 3rd mc, will bench bleed and try again

 

Mark

[mark69sportsroof 31]

Removed the mc. This time it was a pain and fought me for an hour to get it off. Best I can tell the push rod was forward and getting in the way. Got the new one on but my questions is the front of the mc is hitting the shock tower. It looks basic level.

Is the mc suppose to touch the shock tower?

 

 

I'm thinking by hitting the shock tower and is slightly elevated might be causing the leak at the mc and brake boost. I measured the mc's and they measure 7 1/4"

 

Thanks Mark

[Ridge Runner 1,112]

They are close but it should not hit ,is the booster bolted all the way down ?

[mark69sportsroof 31]

It's bolted down tight. It has to slide down the shock tower the last inch or so. I thought about trying to put a small dent

in the shock tower to give it some room. Any other ideas?

 

Thanks Mark

[Ridge Runner 1,112]

I wonder if the mounting bracket is bent ,it should not hit 

[mark69sportsroof 31]

The brake boost and the mc appear to correct and no damage.

[mark69sportsroof 31]

I was wondering if the push rod that goes into the me is to long. When in place does the push rod go all the way into the mc?

Or is there a space at the end the of the push rod and the inside of the mc.

 

Thanks Mark

[LoneWolf2U 136]

The rod end should have .002-.004 clearence from the masters plunger when completely installed with no pruessure on the brake pedal.

installing the master try truning the front end slightly towards the eng and slide it on the closest stud.

[skidmarky 50]

Have you cleaned the fluid out of the booster , or is the booster full of fluid?? Your new MC may not be leaking, the booster may be full. Sounds like your pedal pressure is getting better, bleed it again and see if it improves.

[mark69sportsroof 31]

I don't know if any brake fluid has leaked into the brake boost. Is there an easy way to check?

[det0326 179]

I don't know if any brake fluid has leaked into the brake boost. Is there an easy way to check?

yea if the piston/spool in the master cylinder is leaking, just keep an eye on the rear reservoir it should be going down. As MachMark said the fluid that leaked out initially may still be in the booster and when the diaphragm goes forward it causing some to run down between the master cylinder and booster.  

Dave 

[mark69sportsroof 31]

Frustration is taking over. I got the 3rd new mc. Bench bled it and mounted it to the brake boost. I didn't hook up the two lines. I had two plastic tubes that I used to bleed it and I left them attached to the plastic plugs that screw into the mc.

 

Reached up under the mc where it connect to the brake boost and got 3 fingers wet with brake fluid. How could that be since it

was only mounted?

 

I went to take off the mc and can't get it off. I pulled it toward the engine and upward but I can't get it pas the brake boost

bolts. I wiggled and jiggled and nothing. I don't if the push rod is not going all the way back into the brake boost. I can

barely see it when trying to get the mc off

 

Thanks Mark

[det0326 179]

did the studs possible screw out of the booster some when you removed the nuts?  Replacing a mastercylinder shouldn't be this hard. You may have to remove the booster at this point and see if there is any brake fluid in it and if so try to some how get it dried out.

 

Dave