What's wrong with my master cylinder?

[rogerowen]

Ongoing brake problems have led me to suspect that the master cylinder needed new seals. Job done, bled system - but pedal goes to the floor. Checked everywhere for a fluid leak, but none found. Re-removed master cylinder to check all new seals fitted correctly, found OK, put back, bled again - but still no pedal pressure. What is strange is that when I press the brake pedal I get 'Mount Vesuvius' from the reservoir - I'm talking huge internal brake fluid fountain! Seems that rather than push fluid to the brakes, the piston is pushing the fluid back up into the reservoir.

Anyone experienced this before / any ideas?

Thanks.

[bmcecosse]

There is always some 'blow back' of fluid causing a small fountain.......not sure about Vesuvius though - never seen it.......... Frankly I would not attempt to re-seal my master cylinder. If/when it fails I will get another cylinder. When you do the bleeding- does oil come out at all the cylinders ? Are all the shoes adjusted up TIGHT ?

[rogerowen]

Yes, bleeding seems quite normal and adjusters are as tight as I can get them. Have also tried both sequences of bleeding, start with furthest (as per work shop manual), and start with nearest (instructions on my bleed tube kit). I'll probably have to bite the bullet and purchase a new master cylinder. Gruel for the next month or so!

[bmcecosse]

What was the original problem that caused you to suspect the master cylinder? To test the master cylinder - you need to isolate it and fit bleed nipples into it's outlets. Obviously - it should then be SOLID! Also -when bleeding - release the handbrake cables so they are slack, and then adjust up the rear brakes fully.

[rogerowen]

Original problem was total brake failure (emergency stop with engine braking and hand brake) - V Scary!!! Anyway, checked reservoir and found it to be empty. Couldn't find a leak anywhere although there was a lot of brake fluid sloshing around the master cylinder in the chassis rail. Didn't slack off handbrake adjustment - but hand brake was fully off during bleeding.

[bmcecosse]

If the cables are too tight - they hold the rear pistons forward. Better to slacken them right off and then run up the rear brakes with the snail cams.

[rogerowen]

Many thanks, will give that a try.

Cheers,

Roger

[kennatt]

honestly roger take BMCs advice and get a new cylinder,from my expirience of minor master cylinders,each one I have seen that have failed, have had corrosion on the bottom of the bore,caused by water that gets into the fluid,and sits in the bottom of the bore,even after honing the bore,unless you are VERY lucky,there will be a rough spot which very quickly damages the new seals,and you are back where you started,they are such a pain to get out,better to fit a new one and forget about it for another 20 years,rather than wonder how long it will last,But it is tempting to fit new seals when the costs are compared,but good luck with it, it may be ok for you.

[rogerowen]

Thanks for the advice, bit the bullet and ordered a new one.

Cheers,

Roger

[PAULJ]

Hi, have you taken the drums off and checked that the wheel cylinders are all ok ?

[bmcecosse]

^^^^ Indeed - that's a basic check that doesn't seem to have been mentioned !

[Dean]

Thanks for the advice, bit the bullet and ordered a new one.

Cheers,

Roger

Good decision, at the end of the day it stops you hitting something painful at speed. Best not to scrimp in my opinion.

[rogerowen]

Yes, I checked all wheel cylinders (all fairly new), no leaks there, and of course I've also checked every pipe joint that I can see. Replaced master cylinder today - run out of daylight / got very wet - will bleed tomorrow and post if this sorted the problem.

[bmcecosse]

You missed the GP Qualy and The Tennis to get wet and dirty???

[rogerowen]

Yes! And even drove 100 mile to collect another Moggy to rescue, that's why I didn't get the brakes bled on Moggy No.1. Last time I had the opportunity to watch sport on television the names Ash and Hill were the stars, I believe it's in colour these days!

[bmcecosse]

Through the rain clouds and puddles - yes!

[rogerowen]

Any more of this wet stuff and I'll need a mask and snorkel to fix these brakes! When I win the lottery I'm going to buy a workshop. Actually, will I need one? ............................Yeah!

[rogerowen]

Roll on summer![frame][/frame]

[rogerowen]

New master cylinder fitted - and all working well (for 7" drums anyway!). I'm totally amazed that just a little pitting on the bottom of the master cylinder could have rendered the braking system completely inoperative. Onwards and upwards!

[bmcecosse]

Well done.....I hope it dried up for you!