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Discussion Starter #1
Hello everyone! So I've been experiencing this braking issue and just wanted to see what others think where the issue could lie. So the brake booster and master cylinder was replaced with a new Bendix booster & MC about 5 years back. When I step on the brakes, the pedal stops at the normal height but if I stay on the brakes, I can feel it sinking slowly. It's also evident when the Mustang is at a decline. I can feel it creeping down a little bit and have to press harder on the pedal. I've recent flushed the fluids to see if that fixes the issue but it didn't. Could it be a leak in the booster? I didn't noticed any leaks by the bleeding valves when I was bleeding the brakes. I don't hear any hissing sound like people mentioned if there's a leak in the booster. The valve from the engine vacuum to the booster? Anything else that it could be?
Appreciate any help! Thanks!
 

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Internal seal leak in the master . No fluid loss , fluid leaks by the seal on the piston.
 

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Also, get under the dash and peel back the rubber boot on the back of the master cylinder and check for fluid. Sometimes the boot will hold the fluid of a leaking master cylinder.
 

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Ditto on the master cylinder. To isolate certain parts an pieces on vehicle brake systems I have taken fittings that screw onto or into the suspected culprit. Sometimes you can get plugs that will seal off that circuit other times I've just welded the ends shut.
 

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Discussion Starter #8
So thinking about it, maybe at one time I may have bled the brakes and allow the fluid to run out couple years back. Could it be air in my MC and needs to be bench bled again?
Also about bench bleeding. Why can I set up the bleeding hoses while the MC is already installed and just pump the brake pedals? Why does it have to be done while out of the car?
Thanks!
 

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If you've got "that sinking feeling" in the brake pedal and you have no sign of an external leak, the fluid is probably passing the seals on the master cylinder and ending up in your booster. You can remove the nuts holding the master and just slide it forward a bit to check without having to un-do everything...
 

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Discussion Starter #10
If you've got "that sinking feeling" in the brake pedal and you have no sign of an external leak, the fluid is probably passing the seals on the master cylinder and ending up in your booster. You can remove the nuts holding the master and just slide it forward a bit to check without having to un-do everything...
Thanks Bart! What am I looking for when I slide the MC out a bit? If there's fluid, the the MC, then it's bad and replace?
 

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So thinking about it, maybe at one time I may have bled the brakes and allow the fluid to run out couple years back. Could it be air in my MC and needs to be bench bled again?
Also about bench bleeding. Why can I set up the bleeding hoses while the MC is already installed and just pump the brake pedals? Why does it have to be done while out of the car?
Thanks!
No. Air in the system will make the pedal feel spongy but it will not "bleed" down.
 

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Discussion Starter #14
No. Air in the system will make the pedal feel spongy but it will not "bleed" down.
Thanks for the info! Definitely not spongy.


So in the case that I need a new MC, why can't I bleed it after I installed it and use the bleeding hose kit instead of doing it on the bench which I don't have?
 

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So in the case that I need a new MC, why can't I bleed it after I installed it and use the bleeding hose kit instead of doing it on the bench which I don't have?

You can do exactly as you stated it.
I don't even do that. Just put the MC in place, connect the brake lines and do the 2 man brake bleed. That gets rid of any air.
 

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Discussion Starter #16
Thanks awhtx! Just wondering the reasoning why bench bleeding is all the rage when bleeding in car is so much easier? It was such a pain to semi bench bleed the MC when I replaced the booster and MC 5 years ago. Had to do it on my lap pushed against a wall due to no bench or vice.
 

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Thanks awhtx! Just wondering the reasoning why bench bleeding is all the rage when bleeding in car is so much easier? It was such a pain to semi bench bleed the MC when I replaced the booster and MC 5 years ago. Had to do it on my lap pushed against a wall due to no bench or vice.
It seems some MC's/brake systems can bleed easier then others. Bench bleeding a master is a blanket procedure for all as some can be a pain sometimes, especially when you start adding in cross fed brake systems, weight sensing porportioning valves, ABS, etc . I've put master's on without ever bench bleeding and only bled at the brake line connection at the master once it was installed an had great pedal, I've also bench bled masters and had to bleed at all 4 wheels too before. Just how it goes sometimes!
 

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Discussion Starter #19
So I bought the MC and didn't even check for the leak. It was cheap enough and couldn't hurt if it wasn't the cause but looked like it was by all the comments.

They always take a lot longer to do than first thought. Didn't realized I was going to have to take the booster pretty much out or enough to turn it to make room to remove the MC. Installed, hooked up the lines and started bleeding rear and then front. My front weren't bleeding no matter how many times I pressed on the brakes. Then it turned out that my Earl's Speed Bleeder weren't allowing the fluids to come out unless I open them almost all the way out, then it allowed fluids to come out but also allow air to enter. Went to the store to get more fluids and new bleeding valves. Anyways, all bled and road tested and that fixed my sinking brake issue.

I then realized that I will need to replace my water pump. It started making a grinding sound about a week ago and I was starting the car while bleeding to turn the wheels and to charge the battery and one time noticed steam appeared after shutting the car off. It didn't come back but rather I replace it before it totally goes out.

Thanks for all the inputs! On to the next project.
 
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