Brake problems

[Chris Edgar]

I have a persistent problem with the brakes on the Traveller...

it appears that air is getting into the system, but no fluid is being lost.
The result is a pedal which, over time, sinks lower & lower to the floor. The pressure can be restored for a while by pumping.

I've twice had it to the dealer (still under warranty).
First time they bled the brakes & it was OK for a few days, then deteriorated.
Second time they replaced all four front cylinders & bled the whole system. Same result in the end.

I can bleed the brakes & it will be fine for a few days, then back to square one.
There is no sign whatever of any fluid loss anywhere. No sign of a leak & no need to top up the m/cyl.
All shoes, drums etc. are very new & in good order

Anyone any ideas as to this before I trail back to the dealer? Again!

Chris

[mike.perry]

I would suggest that the master cylinder seals are leaking and brake fluid is getting past them

[Neil MG]

I believe Mike is absolutely correct.

What you describe is not a bleeding or air issue but a leaking seal inside the master cylinder. I am very surprised that the dealer didn't check that. New seals, or new master cylinder if the bore is pitted, and then you get to do the bleeding again! At least you will then have all fresh seals and fluid! Unless the hoses are new I would do those at the same time.

The reason it's not leaking is that fluid gets past the cup seal, but not much past the ring seal (not under the same pressure) and so flows back into the reservoir. You may notice a small amount of fluid inside the dust seal and on the push rod though.

This is a fairly common failure mode.

[rayofleamington]

If bleeding helps to alleviate the symptoms then surely air must be coming out when it's bled.

The air has to get in somewhere and as mentioned, the m/c secondary seal is one of the likely places. However if the secondary is leaking it is much more normal to have a noticeable loss of fluid.
If a wheel cylinder is leaking, it's more likely to get air in without a major loss of fluid (although it's not impossible to be the m/c)

Did this problem occur gradually over time or has it come about after some work was done on the car? If so, what work was done?

[bmcecosse]

I really do think it's a failing master cylinder - get them to fit a NEW one - don't let them just fit seals...... If it's under warranty - let them sort it out - and the more new parts they are willing to fit, the better for you. If they want to try new rear cylinders for example - tell them to go ahead - but make it clear that if that doesn't solve the problem - you will be back fo a master cylinder!

[charlie_morris_minor]

another vote for replacing the master cylinder

[Neil MG]

Apologies Ray, I didn't read the original post carefully enough. I thought that the pedal sank gradually to the floor when the brakes were held on, but now realise that "over time" means a few days.

Now reading more carefully I am a little bemused, it does sound like air is getting in, but no fluid getting out - that does sound unusual.

[Chris Edgar]

Thanks for the replies.
Yes, air must be getting in because when I do the bleeding, quite a lot of air will bubble out both front bleed nipples before it runs clear.

The problem started probably about 3 - 4 months ago, I now realise, & has very gradually got worse. At first I put the increasing pedal travel down to a need for adjustment, also at that time I put new shoes on the front & when adjustment & new shoes did not cure it, took the car back.
They bled the brakes, result a good firm pedal & we thought that that was it.

After a few days it started again. I examined the cyls & considered that there might be very slight weeping from 2 of the front ones.
Took the car back ( last week). They replaced all front cyls as, I have said.

But by yesterday I had excessive travel & bled them again. Now OK for the present.

Clearly I will be taking it back & would be glad of the benefit of expert & experienced opinion beforehand.
I am wondering if I should arrange to leave the car with them until we are certain that it is sorted rather than the job being done "while you wait" as up to now.
Nuisance though. It's hardly just down the road.

regards to all,

Chris

[bmcecosse]

The air must be getting in somewhere - most likely at the master cylinder.

[Chris Edgar]

OK, thanks.
I will report back in the fullness of time...be next week though, I think, before I can take it down.

regards

Chris

[bmcecosse]

It's questionable if you should be driving a car with known defective brakes........take special care. Imagine if they fail and you run into a bunch of skool kids......... I would get them fixed right away.

[simmitc]

My gut feeling is master cylinder, based on having seen similar symptoms in the past.

It is worth checking ALL unions, including the fitting under the brake light switch. Also double check the flexible hoses. Given that you are getting air out of the front but not the back then logic suggests that the fault lies at the front - but it is possible that the air is moving along the pipes from the m/c.

I previously typed a caution about driving with dodgy brakes, but the post seems to have gone AWOL, so I just echo what BMC has said. As your fault seems to develop over time when the car is being driven, leaving it with the garage won't help unless they drive it every day. Whoever drives it needs to take great care, drive to the distance that you can stop with the handbrake "just in case".

[bmcecosse]

The problem is that it has been discussed on a public forum......so God forbid, if anything did happen...... those of us who have posted on the thread could possibly in some way be seen to be liable.........

[simmitc]

Absolutely. My advice would be to drive off road on a suitable test track.

Another thought as to where the air might be coming from: It's a long shot, especially as the wheel cylinders have been replaced, but are the brakes binding, generating heat, and the fluid is boiling? It has been known before.

On the subject of fluid - is it standard or silicone? The former is more prone to boiling as it absorbs moisture, some people have reported problems with bleeding the latter, although I have always found it to be OK.

[kennatt]

have you taken the cover off above the master cylinder and if so is the inside of the chassis leg dry,if the master is leaking this is where the fluid ends up and can take a while before showing itself on the outside of the chassis