How to tell if it's the master cylinder...

[JaneHibbert]

so with my husbands help we checked every cylinder and nipple and line and everything and bled the brakes four times and still when we pump it goes hard for a while and then the pedal just goes down to the floor! Is there any cinclusive test to see if its the master cylinder before i change it?

[mike.perry]

Are you sure that there are no leaks, split pipes, loose connections, leaky wheel cylinders?
Check for brake fluid around the master cylinder.
Pump the brakes up hard and keep pressure on the brake pedal. If it gradually sinks to the floor then the master cylinder is faulty.
I hope that your husband keeps you supplied with cups of tea

[JaneHibbert]

Hi, not gradually no, but as soon as you let it go for more than a couple of minutes it goes to the floor again. We have, as I said before, followed instructions given on here on how to bleed the system and have done it several times with the same issue. Once pumped, if you keep your hand on it HOW gradually would it go down to indicate a problem?

[JaneHibbert]

oh and my husband refuses to make me tea!

[JaneHibbert]

one thing it might be, the screw lid on the master cylinder is REALLY chewed up, it screws on okay, but not sure if it is 'sealing' the master cylinder or not... or if it IS supposed to have ventilation whether the hole in the lid is allowing air out!

Might be nothing of course!

[bmcecosse]

Are all the brakes adjusted up tight?

[beero]

It does sound like adjustment as Roy says. You say you get a hard pedal.
Air in the system normally gives a 'spongey' feel to the pedal as the air bubbles compress.
Try adjusting all the shoes till the drums are locked on and the see what happens with the pedal.

[mike.perry]

Push down on the brake pedal with your foot not your hand.

[ASL642]

I had this - all air removed. Brakes hard but faded to soft. The seals on the MC had perished.

[rayofleamington]

one thing it might be, the screw lid on the master cylinder is REALLY chewed up, it screws on okay, but not sure if it is 'sealing' the master cylinder or not... or if it IS supposed to have ventilation whether the hole in the lid is allowing air out!

The lid has a small vent hole - it is only there to keep muck out and fluid in, but definitely NOT sealed.

9 times out of 10, a failing m/c will give a hint - you get air bubbles coming back up into the reservoir.
Unfortunately it's not a great idea to look for them unless you are VERY careful. The first small part of piston movement releases
fluid back into the reservoir.. making a brake fluid fountain if you move the pedal quickly! You don't want an eye full

The air bubbles occur because the secondary seal is failing. This seal operates in the 'open' end of the m/c so can be affected by dirt, wear and/or corrosion on the cylinder. with a failing secondary, you will get some air into the brakes however much you bleed. Pumping the pedal compresses the air until the pedal feels firm. Waiting without pressing the pedal allows the air to decompress and you are back to square one.

Much less likely (but just as important) is a failed primary seal - these can fail from age/wear/wrong fluid. The primary seal is the one that actually creates the pressure. These can fail in a few ways:
1) so that even when pedal is 'firm' the pedal still moves slowly to the floor (small leak under pressure - damaged seal or scratched bore)
2) you can't get pressure unless you stamp on the pedal (seal does not energise unless there is some pressure - worn seal)
3) no brakes at all - most likely on a frosty morning first use!
4) combinations of 1/2/3 or intermittent occurrences of any.

The description fits having air in the system (ruling out a failed primary seal).
This sounds like failing secondary seal in the m/c, but it could be other issues that make the bleeding process difficult. For example, it's known for new rear cylinders not to self-bleed..

[JaneHibbert]

so it seems like it could be anything, poo.

How do you know if your new rear cylinders have self bled or not?

i think at the weekend ill nip up all the cylinders tight, check all conectionsoesn't do the trick ill get a new MC.

[bmcecosse]

Are you seeing any air come out when you do the 'bleeding'? Slacken off the handbrake cables then tighten the cam adjusters up on the rear brakes. Can't see why they wouldn't bleed - a 'myth' I think that they won't bleed! You aren't using DOT 5 fluid - I hope.......

[JaneHibbert]

not much air no, only what seems to be in the pipe to start with. no it says dot 4 on the bottle. ill just try again to see. want to do it now but ill have to wait til weekend and some more free time.

[IslipMinor]

new rear cylinders have self bled or not

Jane,

What do you mean by 'self bled'? None of the cylinders will self-bleed, they all need bleeding in the conventional way, starting with the furthest from the master cylinder, the n/s rear, then o/s rear, then n/s front and finally the o/s front. Do you have a servo, there may be a separate bleed nipple on it, in which it is the last to be bled.

Unless you have a pressurised bleeding kit, or non-return type nipples, it's a 2-handed job, with the first person opening the bleed nipple by 1 turn or so, the second person pushing the pedal down, the first person tightening the nipple at the bottom of the pedal stroke, second person releases the pedal, first person opens the nipple and repeat the process until no air bubbles come out of the bleed tube. You can normally bleed one wheel without topping up the master cylinder so long as there are not excess bubbles. The pedal must be at the top of its stroke when topping up the master cylinder.

It's a good exercise in communication - you HAVE to talk to each other!!!

It should be possible to calculate how many pumps can be done between top-ups??

[JaneHibbert]

with self bleeding i was refering to what somebody else had said...

[IslipMinor]

Jane,

How have you bled the brakes?

[bmcecosse]

Up here in Ecosseland the gravity/cup of tea system of bleeding works well ! But if in a hurry -the two person/down open/close up system works without fail.

[lambrettalad]

TEA in the land of the, water of life, as Victor Meldrew says "I don't believe it "

[kennatt]

I think you are being overloaded with tech.advice,and will now be totaly confused. Just try this and re post, pump (Press it rapidly a few times with your foot ) the brake and when you get a firm pedal just press as hard as you can and hold it ,for as long as you want,or can as if you were doing an emergency stop.If the pedal stays hard and dosen't slowly sink to the floor whilst holding the pressure,then the M/c is probably OK,and your fault lies in poorly adjusted brakes.If youir foot sinks to the floor the yes its the M/S or a leak in the pipework or wheel cylinders.All air in the system does is to cause a spongy feel,if you get a hard pedal then the air is out.All the bleeding in the world will not produce a hard pedal at the first press if the brakes are not adjusted properly.

[JaneHibbert]

I'm going to do it again on Sunday morning and report back