Master Cylinder

[Orange_Octopus]

Hi everyone,

This is probably a bit of a stupid question but I've been puzzling away and would just like a plain English answer:

What does the master cylinder do and what does the brake servo do?

I've got brakes that don't work on my van (no pressure when you press the brake pedal) and I know it's not a leak from any of the pipes etc as I've checked more than once. Someone suggested it was the master cylinder that needed replacing and another person the servo. Now with a difference of £80 between the two replacement parts I don't really want to get it wrong!

It's probably really simple but every time I try to work it out I confuse myself (not hard ). I'm almost certain it's the master cylinder I need to replace
Thanks
Amber x

[Naadeslaus]

Have you check'd the brakefluid?And tried to get the air out of the system?
Servo?? Is there servo on your van??

[alex_holden]

The master cylinder is the thing the brake pedal pushes on, which forces hydraulic fluid down the brake pipes to the slave cylinders on the wheels. It's underneath the driver's feet on a Moggy.

A brake servo is a thing that uses vacuum from the engine's intake manifold to amplify the force applied to the master cylinder when you press the pedal (AKA power assisted brakes). I've never seen one on a Moggy.

Don't buy any replacement parts until you know what the actual fault is.

[Orange_Octopus]

I've flushed the system 3 times with brand new brake fluid and asked the garage who did the respray to try too while it was up over the "pit" so I'm as certain as a certain bear that the fluid's fine.

Now that's interesting because I don't think I have a servo either!! I think the person who said "servo" was actually getting a bit confused and meant master cylinder (as we'd been talking about that previously) but as I said I wanted to make totally sure. I felt it was the master cylinder and it was only when servo was mentioned I began to doubt myself.

Guess perhaps I should have more faith in my own knowledge!

[bigginger]

If you have got a servo, then it's been retro fitted, and is fairly obvious - a largeish (1ft x about 9 inches) cylindrical unit with pipes leading to and from it, somewhere in the engine bay, generally beside the starter.

[alex_holden]

I think the person who said "servo" was actually getting a bit confused and meant master cylinder

Probably because on modern cars the two are usually fixed together as a single unit.

I bet my brakes are in a worse state than yours The previous owner started to rebuild the system years ago but only got part way before he gave up and left it in the garage!

[Willie]

It should be easy enough to prove if the Master is at fault. If you have flushed it through three times then it must have pumped fluid out of the bleed nipples
on each wheel? assuming that you bled in the correct order i.e. rear N/S,
rear O/S, front N/S, and then front O/S then you should have finished up with
a firm pedal. If not then it is possible that the master is faulty and is able to
pump whilst a bleed nipple is open but fails as soon as the nipple is closed and full pumping pressure is required. When you depress the brake pedal
does it improve if you pump it quickly?

[bmcecosse]

You really need to pull the drums off and see if any of the wheel cylinders are leaking. Master cylinders fail very rarely - but are often needlessly replaced! If you pump the pedal up fast and then keep pressing - does it sink to the floor ? If so - it's leaking somewhere.

[Orange_Octopus]

Sorry - I wasn't very helpful in my description of the problem was I?

I'll try and keep it short - the car has been in a garage for 24 years and although everything runs the brake pedal had no pressure when it finally came out of hiding. The brakes were bled (correct order and fluid came out/went in) and afterwards I had pressure on the brake pedal and the car stopped (always good! ) but when I went back the following weekend the pedal was loose but no fluid could be seen under the car (don't understand that!) so we repeated the proceedure with the same results. As the car was due to be picked up for it's respray I asked the garage to try bleeding them while it was up in the air, which they did (or said they did ), but when the van came back to me the pedal was floppy again.

From what Willie says it sounds like it isn't the Master C then! The pedal only has pressure after the brakes have been flushed. It keeps that pressure for a while, regardless of how fast the pedal's pumped - which would point to a leak but I really can't see one in any of the wheel cylinders or pipes. I still don't understand where the fluid is going but I can only assume it's localised wet weather over the area the van's parked!!!
Thanks for any help you can offer!

[Kevin]

can only assume it's localised wet weather over the area the van's parked!!!
Even so Brake fluid would still leave oily marks.

AKA power assisted brakes). I've never seen one on a Moggy.

Well there have been quite a number at the rallys I have been to this year so its a fairly common addition.
The Servo was given as a culprit I expect as they can consume brake fluid without leaving a trace I assume you definatly havent got one ?
The other place is to have a good look in the chassis leg where the master cylinder is sitting as this may reveal a puddle of brake fluid.

[Willie]

The favourite place for the fluid to escape without showing up is from the
end of the master cylinder(the end with the rubber cover) from where it leaks
into the chassis leg. Obviously this will be accompanied by a drop in the level
in the reservoir. If this is happening then at least an overhaul kit is needed for the Master although I would prefer to fit a new unit if it has lain idle for
24 years! There are two sizes of master cylinder bore(marked on the body
of the master) the early ones being 7/8", the later(1098) ones are 13/16".
Whichever one you have at present I would replace with the later 13/16".

[d_harris]

I was going to suggest the seals in the MC..............