Brake system lack of pressure / fluid throughput

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Fingolfin
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Brake system lack of pressure / fluid throughput

Postby Fingolfin » Wed Oct 17, 2018 6:43 pm

Hi all,
My Mog has been laid up since July with failed brakes. The pedal gradually got spongy over the course of a few days, until all pressure was lost.

I checked each wheel (MGB calipers up front, standard cylinders in back) and had no fluid weepage, and the bleed screws were all tight. No fluid drips were evident from the lines, either, so I immediately concluded that it was failed master cylinder seals. I got a new MC on order (shipping to the US is painful on those!). Upon receipt, I disassembled it to remove the "top-hat" seal to work with the discs up front, as you do - I had done the same thing in 2015 with the old MC, with no trouble - reassembled everything in fluid, put it in the car, and -

Nothing! The system would not pressurize and the reservoir level did not drop. With bleed screws open (on the calipers, the type with one-way valves inside so you can just pump and not have to open and close them), no fluid came through, even after a hundred strokes on the pedal. Some very tiny bubbles would come up through the reservoir with each stroke, though.

After some grousing in the MM Facebook groups I came to the conclusion I must have torn a seal in the MC upon reassembly. So, I got a seal kit on order, waited, disassembled and reassembled, put it back in the car, and the same thing happened. No change in symptom whatsoever.

So I'm rather at a loss. Have I torn the new seals again? If so, how do I make that not happen? Or could it be a defective MC (that is, the metal bit)? Or have I missed something somewhere else in the system entirely? :roll:

Any input appreciated...
The way to a man's heart may be making food, but the way to my heart is buying me car parts!
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shoebone
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Re: Brake system lack of pressure / fluid throughput

Postby shoebone » Wed Oct 17, 2018 7:51 pm

I don't think you have a problem ... once you get fluid moving you will be able to bleed normally, I believe we need the weight of a line full of fluid to get things going and you need to get fluid into the lines, I use a vacuum pump now, seems to work especially well on the minor which has always been problematic with brake system maintenance, previous to that I used to use my hand on the foot peddle and pump like a maniac until the master started pumping !! Good luck.

RobThomas
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Re: Brake system lack of pressure / fluid throughput

Postby RobThomas » Wed Oct 17, 2018 8:11 pm

Have you tried pinching off the hoses to the front cylinders to halve the quantity of fluid you need to move? Once the back is done, push the pistons deep into the calipers to reduce the air. Sounds like you might have so much air in there that the fluid is sort of just bouncing back as you release the pedal.
Might work?
Cardiff, UK

Fingolfin
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Re: Brake system lack of pressure / fluid throughput

Postby Fingolfin » Wed Oct 17, 2018 11:01 pm

An air lock is something I've considered, but I discounted the possibility because fluid doesn't move even when the bleed screws are open (air can't lock when the system freely vents, right...?). But I'll get a vacuum pump and try it out - if it fails, that's just another datum point!
The way to a man's heart may be making food, but the way to my heart is buying me car parts!
Come read about my Minor at An American Moggie.


philthehill
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Re: Brake system lack of pressure / fluid throughput

Postby philthehill » Thu Oct 18, 2018 7:04 am

Try jacking the rear of the Minor as high as you can.
I once had to deal with a Minor with similar problems that needed the car to be nearly stood on its nose to get the air out of the system.


Sleeper
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Re: Brake system lack of pressure / fluid throughput

Postby Sleeper » Thu Oct 18, 2018 8:08 am

Fingolfin wrote:
Wed Oct 17, 2018 11:01 pm
An air lock is something I've considered, but I discounted the possibility because fluid doesn't move even when the bleed screws are open (air can't lock when the system freely vents, right...?). But I'll get a vacuum pump and try it out - if it fails, that's just another datum point!
Let us know how the vacuum pump works,it's on my "to do" list...

John :wink:

Nickol
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Re: Brake system lack of pressure / fluid throughput

Postby Nickol » Thu Oct 18, 2018 9:25 am

philthehill wrote:
Thu Oct 18, 2018 7:04 am
Try jacking the rear of the Minor as high as you can.
I once had to deal with a Minor with similar problems that needed the car to be nearly stood on its nose to get the air out of the system.
Have had similar problems and I came to the conclusion, rightly or wrongly that the rear brakes were the problem with air not being expelled. Normally I bleed the brakes myself but to solve the problem I had a friend do the pedal pumping for me with the rear of the car, like Phil said raised as high as possible. Thence he pushed the pedal down as quickly as possible several times with a pause in between and indeed ( only the rear brakes ) some small air bubbles could be seen in the tube.

Much better now but it was a long odyssy to get there
Gott schütze mich vorm Sturm und Wind und Autos, die aus England sind.
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alanworland
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Re: Brake system lack of pressure / fluid throughput

Postby alanworland » Thu Oct 18, 2018 9:45 am

I made an adapter to screw in the top of the MC which allows a few psi to be applied to the system, then open bleed screws and allow the positive pressure to push fluid/air through.
Don't let fluid get too low!
To me it sounds like you have a big wodge of air which is compressing rather than moving.

Alan
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Fingolfin
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Re: Brake system lack of pressure / fluid throughput

Postby Fingolfin » Thu Oct 18, 2018 4:30 pm

Last night I only had time enough to try the vacuum pump on one of the front bleed screws. Upon pumping up, I got a few drops of fluid out, but never a consistent flow. My thought now is that there may be a blockage in the line going to the front brakes.

Will try rears this evening...
The way to a man's heart may be making food, but the way to my heart is buying me car parts!
Come read about my Minor at An American Moggie.


liammonty
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Re: Brake system lack of pressure / fluid throughput

Postby liammonty » Thu Oct 18, 2018 5:26 pm

Check if the flexis have collapsed internally - i had this problem a couple of months ago whereby I couldn't bleed the fronts after changing the cylinders (on mmy Wolseley Six, but same principle) and it was due to a collapsed flexi that wasn't allowing fluid through on the side that I was trying to bleed first. Hose replaced and all was well.

ampwhu
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Re: Brake system lack of pressure / fluid throughput

Postby ampwhu » Thu Oct 18, 2018 6:02 pm

i would crack a brake union at the highest point in the brake system as well. Slacken it on the brake pipe junction. push the pedal. i'd also check one of the front brake pipes hasn't collapsed internally.

biomed32uk
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Re: Brake system lack of pressure / fluid throughput

Postby biomed32uk » Sat Oct 20, 2018 10:06 am

For bleeding vacuum bleeder all the way, works an absolute treat with no messing. Really easy, whole car can be bled in half an hour, single handed with a perfect pedal.

Sleeper
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Re: Brake system lack of pressure / fluid throughput

Postby Sleeper » Sat Oct 20, 2018 2:39 pm

As soon as I can get round to building the Vacuum bleed kit I'll give it a go....

John :wink:

Fingolfin
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Re: Brake system lack of pressure / fluid throughput

Postby Fingolfin » Mon Nov 05, 2018 6:33 pm

Thanks to everyone who gave me suggestions on this thread. It took four different bleeding sessions, but it did in fact turn out to be an air lock - the pedal is now quite firm and the car stops nicely. 8)

There probably was no need to replace the seals in the MC...but it has been a learning experience, so I should try to be satisfied with that.
The way to a man's heart may be making food, but the way to my heart is buying me car parts!
Come read about my Minor at An American Moggie.



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