Jag XJ6 erratic brakes

A collapsing hose - rare though it is ;-) - would not make the pedal go soft. Nor would a fault on the servo connection - that would make it go hard.

Master cylinders can sometimes leak fluid past the first seal without it leaking out, as it were.

But I'd bleed them all again first - preferably with a pressure bleeder like an Ezibleed. Run about a litre of fluid through.

Sounds like something has seized on and is causing an intermittent fault.

Anyway after a year the brakes should have been serviced and checked as the fluid does absorb moisture. Not using them allows rust to build up hence causing the seized pistons.

Had the same prob with a Scimitar... and a Transit full of school-kids.

There is fluid leaking past the return seal in the master cylinder, instead of it being pumped to the wheel cylinders. A seal-kit might do the job, if you're lucky, otherwise replace the whole cylinder (which is the safest option).

Allan Bennett

Both of the above would cause no brakes on the circuit affected and, as you say, a firmer brake pedal.

Yes, I suspect a master cylinder fault, I suggest that all the brake hoses are clamped off (obviously the car is un=drivable like this...) and then with the engine running keep pumping the brake pedal on and off - there should be a *solid* pedal with little or no travel, if there is then remove the clamps and bleed the brakes (don't skimp on the amount of fluid used either) and re do the test above, if the problem is still there it very much looks like the master cylinder is suspect.

You'll still have brakes even with the servo vacuum disconnected - they'll just be heavy - *very* heavy in the case of a car with discs all round.

Oh, that hose, me was think more along the lines of brake fluid line hoses, not vacuum, sorry if I've confused....!

As for collapsing vacuum hoses, I've seen a few and as you say the pedal would become hard to push but the brakes would still work, just was would happen if the engine cut out whilst the car was moving.

You should still have normal brakes for one stop even if the engine stalls

- and of course if you stay in gear, a dead engine will usually still produce vacuum. Unless the cam belt has broken. Or it's an auto. ;-)

Recuperating seal/valve in master cylinder fluid is\seeping back to the resevoir -- called in the trade a sinking pedal. To check for sinking pedal -- with engine running apply very very gentle pressure to pedal, if pedal very gradually sinks a couple of inches the seal/valve is u/s.

That might not show the fault up, try applying normal braking pressure to the pedal and then *very slightly* release the pressure and then re-apply the pressure, the pedal should not be any further down it's available travel than for the first time pressure was applied, if the fault above is present (also known as a treading down pedal) each slight release and re pressurisation will eventually find the pedal at the bottom of it's travel with no pressure in the system at all.

It was somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Dave Plowman (News)" saying something like:

Last time I drove a car the weight of a Jag with the servo disconnected, I near crapped myself the first time I came to use the brakes.

One of those "OOHHHHHHSHIIIIIITTTTT!!!!!!!!!!" moments. Fortunately I was just moving the shith... car round the corner.

I had the same when I tried to move an XJ40 with the power brakes/steering disconencted. Almost impossible to drive. It made me laugh at the driver's manual which claims that it is possible to drive and brake safely even if the hydraulic pump fails.

It was somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember snipped-for-privacy@malloc.co.uk (Steve Firth) saying something like:

Heh. My old auto Granada died mid-corner on me . Sans power steering it was a lardy old beast. I cured that pronto, since it drove the point home rather forcibly that an auto with an unreliable engine is not desireable at all.

Thanks for the replies, all, though for some reason I can view only a few of the total number in my news reader...

I haven't had a chance to do anything with these brakes yet, though have established that I always have brakes when the car is cold - so based on your replies I'm thinking it's contaminated fluid and/or master cylinder failure.

Well, I got around to bleeding the brakes, put in new fluid and... same as before. So, new master cylinder required - any pointers to a supplier and opinion on if I should go for OE or alternative appreciated.

Not a new cylinder just a set of seals --- repair kit vry easy to fit to girling dual cylinders

Not if the bore is worn, don't cheap skate on brakes, to do so can kill.....

You obviously place little value on life.

I have a lot more experience of real world motor engineering than you together with a pile of accademic quailfications in mechanical engineering backed by recieved wisdom from family motor trade background dating back 100 years so please don't lecture me on what you know little of. Girling dual brake master cylinders are rarely so badly worn that they need replacing the only things to watch for are damage to te spring and the build up of crud at the pushrod end of the cylinder, from the symptoms the cylinder in queston has a recuperating seal or valve problem, a repair kit is an easy and longterm fix. A quick trip ti LSUK for parts and a good experienced mechanic can do the job in 20 minutes without need to bleed the whole brake system

What I've done is order the complete master cylinder - I don't know anything about what cylinders are easily reconditioned vs. those that aren't, etc... I want to fix the car asap, and given my location, I rely on mail order. I didn't want to order seals only to find they don't fix the problem. As the cyl. on the car is likely 20 years old I thought it best to replace it given the relatively low cost. When I've the old cyl. off I will have a good look at the seals, bore, etc. and if it looks okay, I'll order a set of seals to have as a back up.

Question - what's 'LSUK'?

LSUK was Lucas Service UK the partys service of Lucas Girling and Lockhee they have branches or franchised agents in most towns.