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Brake Master Keeps blowing
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hogan029
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PostPosted: Wed Aug 26, 2020 9:47 pm    Post subject: Brake Master Keeps blowing Reply with quote

Hey all,

Been having some brake issues for quite some time. My master cylinders keep failing on me (three in the past year). I just installed one this past weekend when when I was doing the initial bleed, it blew. I have been using the pump and hold method which I feel should be okay as opposed to a pressure bleed for the install.

Anyone experienced this or have any comments on what I should try next?
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dobryan Premium Member
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PostPosted: Thu Aug 27, 2020 5:40 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

What does it do when it blows?
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PostPosted: Thu Aug 27, 2020 6:02 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

What brand? Bleeding the brakes with an assistant pumping the pedal is no reason for a new master to fail.
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4Gears4Tires
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PostPosted: Thu Aug 27, 2020 6:37 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

^ Yeah. Are you getting the same brand each time? If you're getting ebay parts, it may be time to try your luck with 1. ordering from a vendor and 2. buying an OEM level part.

https://www.busdepot.com/251611021c
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hogan029
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PostPosted: Thu Aug 27, 2020 10:54 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

Hey I am using one from a local shop, will get the brand name but not a cheapo one.. what i mean by blowing is that i will pump all the air out and the all of a sudden i will lose all pressure and the pedal gets stuck down. I cant build up anymore pressure. I checked for leaks in the whole brake system and have not found anything
. I'm really at a loss here
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hogan029
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PostPosted: Thu Aug 27, 2020 10:59 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

I should add that the fluid comes out through the seals on the booster side of the master as well
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markswagen
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PostPosted: Thu Aug 27, 2020 11:05 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

check the throw of the pushrod coming out of the booster, then check how far you can push in the center of the master cylinder, maybe you are pushing too much, over extending, and causing damage to the inside of the master cylinder, what ever the brand is, i would try a different brand, i use only CIFAM master cylinders in customer vans.
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hogan029
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PostPosted: Thu Aug 27, 2020 8:50 pm    Post subject: Re: Brake Master Keeps blowing Reply with quote

thanks for the reply. Is there any kind of measurement that I can make for the pin depth?
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PostPosted: Fri Aug 28, 2020 4:50 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

You could always disassemble to see what is failing. Assuming stock parts, you should feel clearance when you first press the pedal with your hand before the pushrod contacts the master. Maybe an 1/8 to a 1/4 free play. If you have that, I donít see how the rod could be too long.

I would gravity bleed first. Then you can do one wheel at a time starting with R/R. The pedal doesnít need to go to the floor. Put something under the pedal to act as an early stop or find another assistant.
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hogan029
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PostPosted: Sat Aug 29, 2020 8:53 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

So I did the gravity bleed as suggested. The pedal is now firm. But... when I start it up, the pedal is all soft and crappy again. Thinking maybe the booster??
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PostPosted: Sat Aug 29, 2020 9:11 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

A good pedal comes from a good adjustment on the rear brakes. You should look into that and not count on the self adjusting feature at this point. Plus, gravity bleeding gets you there, but it's still recommended to manually bleed each wheel starting at the r/r.


It sounds like the booster is working as designed. Its their to assist the pedal.

To get a proper rear brake adjustment, the hand brake cables need to be backed off completely so that the shoe arms are not applying any pressure. The rears are then adjusted, and then you adjust the hand brake.

Many of these vans have had the parking brake adjustment tweaked to adjust the handle. Also dragging hand brake cables can also fight a good rear brake adjustment.
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hogan029
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PostPosted: Sat Aug 29, 2020 4:06 pm    Post subject: Re: Brake Master Keeps blowing Reply with quote

okay that makes sense! to clarify, its hard when the van is turned off and when started, it becomes quite soft.

Is there any written procedure for the rear shoe adjustment?
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PostPosted: Sat Aug 29, 2020 7:56 pm    Post subject: Re: Brake Master Keeps blowing Reply with quote

Did you clean your reservoir?

I`ve seen reservoirs with more mud than the bottom of a swamp...
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hogan029
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PostPosted: Sat Aug 29, 2020 8:35 pm    Post subject: Re: Brake Master Keeps blowing Reply with quote

I actually cleaned it all out when I changed the cylinder
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PostPosted: Sat Aug 29, 2020 9:56 pm    Post subject: Re: Brake Master Keeps blowing Reply with quote

hogan029 wrote:
all of a sudden i will lose all pressure and the pedal gets stuck down


are you doing this running?

either way, that is a sign of a bad booster unless the brake push rod is so out of whack it is jamming the master. there are just too many return springs in the system to even make a pedal stick to the floor possible.

in my 30+ years in a shop the only time I have seen a pedal stuck to the floor is because a booster went bad
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hogan029
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PostPosted: Sun Aug 30, 2020 9:54 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

It doesnt get stuck all the way down, id say like a 1/4 of the way. But I am thinking of doing the BMW upgrade and adjusting the brakes. What is the procedure for adjusting the rear brakes? I have seen a few different ones on my search but unsure of what works best.
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hogan029
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PostPosted: Sun Aug 30, 2020 9:50 pm    Post subject: Re: Brake Master Keeps blowing Reply with quote

I think I may have solved it. Found a leaky brake line where it connects to the rear brake cylinder. Going to replace the line and the cylinder and see.
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Wildthings
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PostPosted: Sun Aug 30, 2020 11:11 pm    Post subject: Re: Brake Master Keeps blowing Reply with quote

Probably time to replace all your rubber brake hoses and if you live in salt country maybe replace all your metal lines as well.
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4Gears4Tires
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PostPosted: Mon Aug 31, 2020 6:20 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

Yep. Time to replace all the brake lines. Do it once and don't worry about it again. If you use Nickle-Copper brake lines, they won't rust out on you and they are very easy to bend into shape.
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PostPosted: Mon Aug 31, 2020 8:27 am    Post subject: Re: Brake Master Keeps blowing Reply with quote

hogan029 wrote:
I think I may have solved it. Found a leaky brake line where it connects to the rear brake cylinder. Going to replace the line and the cylinder and see.


Sure, take care of leaks, but unless it's literally pissing fluid, that's not your problem.
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