Morris Minor Owners Club

Hi,

After replacing all wheel cylinders, I've recently had the master cylinder replaced. The car brakes fine, but if I don't brake for say 30 min when I do the pedal goes to the floor (still brakes but with lots more travel). If I pump it gets back to normal for the next 30 min. Is it likely to be an air bubble in the circuit somewhere? I've checked the brake fluid level and it's still right to the top after hundreds of miles.

What is the best way to bleed throughout and be 100% sure there's no air bubble left anywhere?

Regards

Get someone to help you and do it the 'old fashioned' way .. in, in ... out, out .. in, in ... out, out .. in, in ... out, out .. in, in ... out, out ... ok, lets top it back up again .. in, in ... out, out .. in, in ... out, out .. in, in ... out, out .. in, in ... out, out ... ok, lets top it back up again ...

it would certainly seem that you have an air bubble in your system. If you have bleed the system and were getting no bubbles out check that all your bleed nipples are done up tight and not allowing air back into the system.

One thing I tend to do when bleeding the brakes is not to let the brake pedal up gently I tend to take my hand off violently which "bangs" the whole system and dislodges any air bubbles that might be stuck in the system.

Dont forget to release the handbrake and adjust off all cylinders before bleeding.

And make sure you have an ample supply of coffee, if you end up doing it the "Scots" way

Pretty sure you are just needing to adjust up the brakes. When you use them - the residual pressure valve holds the shoes near the drums - so next time you use them, they are 'right there'. However - if not used for a while, the pressure leaks away and the shoes settle back onto the adjusters. Nip them all up and I'm sure all will be well.

It does sound more like adjustment than air.
If you adjust them until the drums can only just be turned by hand, do you get the same problem?
If you can't get the drums tight by adjusting the shoes (ie the snail cam won't turn anymore) you may have a common problem - the leading edge of the shoe is fouling the adjuster cam. A few mm removed with a file / grinder from the offending part of the shoe will fix it.

If you can get the shoe's tight to the drums but still have the same long pedal that needs to be pumped up, it's one of 2 things
1) faulty new m/c
2) air in system.

Air will give you a long pedal that should remain a bit spongey even when pumped up.
A dodgy new m/c can give any kind of feeling - myself and Cam struggled over one that had a long pedal many times, with no joy. After ~18 months the new M/C was replaced again and finally a good pedal! Fortunately a faulty new M/C is VERY rare (unless you got a pattern one).

sorry to resurect this thread, but my drums are all adjusted up so you can just turn them by hand- but my brake pedal has just half the pedal as free play, then the pedal stops about a quarter of the way off the floor. i consulted my engineering teacher, and he said that there will be some free play, and aslong as it doesn't hit the floor it should be ok. i have entirely overhauled the the braking system, including all genuine oe cylinders (master and slaves) and new pipes and flexies. the system is bleed and the pedal does not build. when the pedal is realeased the shaft is level with the floor. is this just the way the brakes are, or will the pedalbe more responsive as all the new components "bed in"?
thanks in advance

There is an adjustment on the operation rod from the pedal to the master cylinder. The 'free play' should be minimal (but present!) befor the pedal starts to move the master cylinder.

the pedal moves the master cylinder- if i adjust it more it it will be pressing against the toe board, and it will mean the master cylinder is always slightly "on"- is this allowed/advisable?

But you said you had a lot of 'free travel' ?? No - there must be a little free movement - the master must not be ON all the time.

the master cylinder piston moves with the pedal, you just have to displace a reasonable amount of fluid to get noticable brake. the yoke just adjusts the pedal in relation to the master cylinders natural spring return position doesn't it?

Well - it's either air - or adjustment then! Pull all the adjusters up so the shoes are hard on the drums. How's the pedal now ? If still floppy - bleed them like this - then re-adjust to give minimal dragging and road test.

when the car is actually on the road, i can tell you however, surely if there is air in the system the pedal would build- and it doesn't. also, if the adjustment was the problem, you would press it down and it would nearly hit the floor, then second press and it would be right there, again not the case. the pedal remains in the same constant position, every time the brakes are pressed- even if it is a second or a fortnight after they were last tried!

How much wear is there between the pedal and the shaft it turns on?
That shaft can end up looking like a crude crankshaft if it is badly worn.
I believe there may be a brass bush in the pedal itself.

I assume you have taken the cover off the master cylinder and examined what is going on down there.

the shaft is ok, and the bush is alright to. i had to do work on the clutch and took the shaft out and the pedal too. the pedal moves very marginally on the shaft, but not excessively. i have the whole gearbox cover off, and the master cylinder cover plate is not fitted yet. maybe the front drums are worn- apparently a common problem?

Highly unlikely! The adjusters have plenty of travel to take up all reasonable wear.

it shall have to remain as it is for the time being- the shoes are adjusted right tight against the drums. it may work itself out after a few miless of use- there are still sufficient brakes.