Brake Master Cylinder Refurb

[Michaeleff]

Load of tosh, DC!

Just stand back and consider this logically.

The Master Cylinder is simply an hydraulic pump: forget the servo, forget the ABS.

Since you are dealing with a Split System (Front and Rear Brakes separated) it is two hydraulic pumps: one for the Front circuit; and one for the Rear Circuit.

The ABS is plumbed into the front circuit. What is does is detect wheel lock, by the simple process of detecting wheel rotation. When there is existing hydraulic pressure detected in the ABS; and when the wheel rotation detectors signal wheel lock, then the ABS unit starts what is called "Cadence Braking". Simply brakes on and off, sequentially.

In the good old days, we used manual Cadence Braking for rallying: indeed the eminent Dutch rally driver. Rob Slotemaker used to run a school, teaching drivers the art and skill of Cadence Braking.

Here's an old film from the Pathé archive, with Rob showing pupils how to do it (at the very end).

Here:

All you need to do, DC, is source the correct seals and bits. Oodles of suppliers!

Careful with honing the cylinder bore, however! Two pumps in one, remember and usually, two different internal diameters. Mainly since front braking effort is circa 70% of total and rear, obviously, circa 30%.

Called Brake Balance. On race and rally cars it is normally adjustable.

If the cylinder bores are beyond redemption, then there are re-conditioners who will re-bore and sleeve.

If the bores are seriously scored, then pointless fitting new seals as they will wear rapidly if not leak immediately.

[Laird Scooby]

Originally Posted by DCrompton940

Thanks Laird, not going to reply on Haynes. I have redone the brakes back and front, bled, all working, no leaks but pedal still a bit spongy. The MC also makes a groaning noise when you push the pedal, something you can't hear when driving

It's got ABS on it hasn't it? Before you go to the grief of pulling the master cylinder off, take it for a drive along a quiet road where you can slam the brakes on about 30mph hard enough to activate the ABS. Do this a few times, leave it overnight - i think you'll be surprised!

[Michaeleff]

Originally Posted by Michaeleff

The ABS is plumbed into the front circuit.

Can be both. A quick examination of the rear hubs should show.

Expanding; most ABS intervention will be on the front wheel brakes, as they are the most usual to lock up on heavy braking.

[Laird Scooby]

On cars with ABS, the brake circuit is split front/rear and the ABS operates on all 4 wheels. However there is only one sensor on the back end, that is the speedo sensor which splits its signal between the speedo and the ABS ECU and both rear wheels are treated as one and the same wheel for the ABS system.

Something that can cause problems is the bleeding sequence - what sequence was used?

Talking of bleeding, did you pressure or manual bleed it? I'm guessing yours still has the Bosch ABS2E system on it which is notoriously hard to bleed because of the pressure needed to overcome some of the valves in the modulator block.
So far the only method that has worked first time every time for me is pressure bleeding using a Gunsons Eezi-Bleed - mine even came with the right cap for the Volvo reservoir! Still need to make a cap for my other beasts but that's for another forum.

[DCrompton940]

I did a manual bleed but once I get the MC sorted, I will try the Eezi-bleed system. Thanks for the advice.

[bob12]

Originally Posted by Laird Scooby

When was the Haynes Manual you're referring to originally printed? I've come across things like this before where Haynes (or another publisher) has stated a certain task isn't possible but often without giving the reason why.

Usually the reason is the manual was printed in say 1997 and covers the model up to 96 ................ etc.

Nothing to do with the master cylinder but this made me smile. The 1996 and next edition of the Haynes Manuals for the 740/60 range made no mention of LH2.4 injection that was introduced in 1990. The first real detail came out in the 2003 edition some 13 years after introduction

[DCrompton940]

I started this thread enquiring if one could refurbish a brake master cylinder on a 1994 model 940 with ABS. As it turned out, i didn't need to refurbish since one of the forum members, 827Roverman, kindly gave me one that he had spare, which was in good condition. The replacement is fitted, brakes all bled and working.

For those who may be interested, I can know answer my own query and comment on some of the issues raised.

The Haynes manual is incorrect, the brake master cylinder can be refurbished by changing the seals. The only trick is that the front piston is held in place by a steel pin that can only be accessed by removing the plastic brake fluid holder. Other than that it is fairly straight forward with the cost of new seals being £10. If anyone wants to do the job I have some photos that I can send you. Alternatively if anyone is looking for a master cylinder fitted with new seals, please contact me.

Much has been said on the forum about the issue of bleeding the brakes using pressure bleeding rather than pumping the master cylinder. I have always used a bleeding tube with a one way valve pumping the MC, but based on the comments I purchased an easi-bleed system. I bench bled the MC before I fitted the unit and I doubt that any air was introduced into the system but i did bleed the system again using the pressure system. The pressure system worked but I wasn't blown away by it (excuse the pun), I am not convinced that in practical terms it is any better than my usual system, the pressure provided using the MC as a pump is certainly greater.

On a similar note, many comments about the importance of changing the brake fuild and what could happen if the fluid deteriorates. I have been maintaining my own cars on and off for 40 years and to date I am still waiting for an occasion when changing the brake fluid had any impact on the braking and that includes cars where the fluid hadn't been changed for many years. Yes, under extreme circumstances I am sure that many things could happen but my experience is that this is not the norm.

[827Roverman]

Originally Posted by DCrompton940

I started this thread enquiring if one could refurbish a brake master cylinder on a 1994 model 940 with ABS. As it turned out, i didn't need to refurbish since one of the forum members, 827Roverman, kindly gave me one that he had spare, which was in good condition. The replacement is fitted, brakes all bled and working.

For those who may be interested, I can know answer my own query and comment on some of the issues raised.

The Haynes manual is incorrect, the brake master cylinder can be refurbished by changing the seals. The only trick is that the front piston is held in place by a steel pin that can only be accessed by removing the plastic brake fluid holder. Other than that it is fairly straight forward with the cost of new seals being £10. If anyone wants to do the job I have some photos that I can send you. Alternatively if anyone is looking for a master cylinder fitted with new seals, please contact me.

Much has been said on the forum about the issue of bleeding the brakes using pressure bleeding rather than pumping the master cylinder. I have always used a bleeding tube with a one way valve pumping the MC, but based on the comments I purchased an easi-bleed system. I bench bled the MC before I fitted the unit and I doubt that any air was introduced into the system but i did bleed the system again using the pressure system. The pressure system worked but I wasn't blown away by it (excuse the pun), I am not convinced that in practical terms it is any better than my usual system, the pressure provided using the MC as a pump is certainly greater.

On a similar note, many comments about the importance of changing the brake fuild and what could happen if the fluid deteriorates. I have been maintaining my own cars on and off for 40 years and to date I am still waiting for an occasion when changing the brake fluid had any impact on the braking and that includes cars where the fluid hadn't been changed for many years. Yes, under extreme circumstances I am sure that many things could happen but my experience is that this is not the norm.

Glad to have been some help to you, David.

[Michaeleff]

A little story that I hope will interest many of you.

I left Ford Europe in 1968 to start my own automotive business. It was a most interesting time in automotive engineering and motor sport. However, I needed to feed my wife and new addition; my son.

A good chum came into to me one day; he was a dealer in anything, had made loads of money and was still grafting. He offered me a large parcel of Trico screenwasher bottles, complete with down tube and foot valve.

Working on my own much of the time, I had problems bleeding brakes. So I invented a brake bleeder; by simply changing the bottle foot valve (it allowed sucking but not blowing! and I need one which allowed blowing. The reverse if you like. I sourced one; made by a company called Trafalgar for BMC 1100s. One must remember in those far off days, a screenwasher was pretty rare: and really a sort of water pistol! The original Mini (screenwasher extra) had a rubber bulb to squeeze! Pumps came later and electric pumps later still.

The Trafalgar valve was made from brass, had a glass ball which seated on a concave inner rim. The valve simply pushed into the poly tube.

Point of fact I then sold all the Trico footvalves and down pipes to another chum who ran a car electrical business (rebuilding starters and dynamos and CV boxes etc and sold a range of parts). The valves and pipes returned more than I had paid for the complete bottles!

The tool worked well, so I decided to sell them. Needed a whizzy name. I came up with OMO Eezibleed. (One Man Operated). Had labels printed up and started advertising.

My business expanded explosively and by June 1969 I purchased a garage retail site. No time for the Eezi-Bleed. Swapped with a nice young man starting out, for a nice MGA 1600 MK II Convertible. The buyer had taken a workshop including equipment from another guy (who became a very close friend and also deeply involved in precision engineering and racing).

The young guy struggled so my other friend took back his workshop and canny lad, sold the Eezi-Bleed to Gunsons then in London's East End and also a newish company. Gunsons changed the product to an air-driven force feed from a spare tyre as we know.

I still have some of the Trafalgar valves somewhere in my sundry parts bin.

In the loft amongst my memorabilia, I probably have some spare labels for posterity. Later this year Mrs MF and I are clearing this (Bloody needs it!) and if I find them will scan and post for fun.

BTW: I do not like the concept of driving the brake fluid using air; can aerate the fluid. The very best way to bleed is using a Vacuum device. Either needs a compressor (small one is fine); or a hand vacuum pump.

Example:

I bought one a year or so ago, since I needed to test my Fuel pressure Regulator. Useful bit of kit. Need a reasonable job with a depression gauge to be any use, though.

A decent depression in the system is usually sufficient to "crack" ABS and Balance Valves. Which is why garages use the vacuum bleeder these days.

By the way DC: brake fluid is Hygroscopic; i.e. it absorbs moisture from the atmosphere. The danger is under constant heating (high speed run in Summer perhaps) the temperature of the fluid rises enormously (All hydraulic fluid does; which is why serious hydraulic kit (Earth-Movers etc) have intercoolers between the hydraulic pump and the fluid return feed.

If you have water in the brake lines this boils and since steam is compressible - no brakes! The first law of Hydraulics relies on the fact that fluids are virtually non-compressible; thus the energy imparted to the prime mover is transferred almost 100% to the slave.

Furthermore, brake fluid degrades over time.

Every workshop manual and practice data demands fluid be changed, periodically. Indeed, Jaguar used to recommend draining and flushing with alcohol, the whole system, and changing ALL seals; every four years.

[DCrompton940]

I am aware that brake fluid is hygroscopic, thats not the issue, its the rate at which absorption takes place. On heavy duty machines and racing cars I am sure its a critical issue, my experience is less so in a normal car. I am sure that Jaguar would like you to change as much as possible as soon as possible, its good business.

As far as bleeding the system, I am sure there is always a system which is technically superior, for me the pressure system didn't do anything my normal system didn't.