Spongy Brake Pedal...Read On???

Has anyone experienced this brake problem before, as I’m stuck???
Just can’t clear a spongy brake pedal?

Father-in-Laws series II, was still running it’s original 10” brakes all round, but had a servo and dual circuit master cylinder fitted from a late series III six or seven years ago. (no brake shuttle safety valve fitted)

The brakes were crap, but have always worked OKish, and it’s just passed its MOT this year, again without any problems, even with the said crap 10” brakes?
However, when checking it over for it’s MOT the brakes really looked like they could do with an overhaul, so I suggested fitting late SWB series III (& LWB) 11” drums on the front, with the wider shoes and twin cylinders, and fitting late SWB series III (& LWB) bigger diameter cylinders to the rear brakes (but still leaving the 10” drums)
I had a complete front end from an 1982 Series III SWB which had new cylinders fitted about four years ago, so the backplates, hubs, pipework etc were all swapped across, I fitted new seals to the cylinders “just in case” as although they “were” new, the cylinders had been sitting for a while? new shoes were fitted on the front, new shoes and the bigger dia. cylinders were fitted to the back?
Problem came when we tried to bleed the brakes?
The back bled no problem, but we’ve struggled to bleed the front?? there seemed like endless amounts of air came through, it would run clear for a few pumps, then a great splurge of bubbles would come, but even when it eventually cleared, we were still left with a spongy pedal?
We were told to change the pipework on the front cylinders, to move the bleed nipple from the bottom to the top, this was done, but after endless attempts at bleeding, still we had a pedal which went almost to the floor on the first push, then firmed up on the second push?
So onto eBay, I bought a Gunsons pressurised “easy bleeder” thinking this would help purge through any air, but it’s made no difference, even with it on “and” pumping the pedal, there’s no air coming through, but the pedal still goes to the floor on the first push?
If you take it for a drive, and brake hard, the back wheels lock, and the fronts seem to be doing nothing? (which is quite scary, not advised!)
The only thing I could think was that seals were gone in the master cylinder, maybe the piston that operates the front brakes was stuck at the bottom of the cylinder, and that’s why we were only getting pressure to the rear brakes?
So a new late series III master cylinder was purchased yesterday and fitted last night?
The back bled again no problem, front was a complete pain, struggled to get all the air out, we ditched the Gunsons easy bleeder and went back to some serious pedal pumping, eventually it ran clear of air, checked it several times, all OK, but the pedal is still the same, almost to the floor on the first push, firms up on the second, and on a test drive, the back wheels lock, the fronts just keep rolling on? (if you check the front wheels while jacked up, they do work, they just don’t seem to have much power)

The master cylinder is piped with the front brakes going to the front of the cylinder, (the big nut) this is how it seems to be shown in the Haynes manual, but is this right?

Master cylinders… have we got the right one? early 90-110 units look identical to the series III units I’ve seen, but are they different for the disc brakes? And would this cause a problem?

Brake hoses weren’t new but looked ok, clear to blow through, and if you hold onto them (which I know your not supposed to do) they don’t appear to swell while under pressure?

Any help gratefully appreciated, as I have to admit, at this point I’m well and truly stuck??
It’s not rocket science, but it’s got me beat!!!!

Cheers

Ken.

split in the servo diaphragm - or did i miss you saying it's been changed

Hi,

If you examine the front wheel cylinders. you will see that the bleed nipple is on the centre line. This means that there will be a pocket of air above the bleed nipple that you cannot get rid of.

Solution is to wind the adjusters right OFF, which allows the pistons to fully retract, thus reducing the size of the air pocket. Bleed brakes ensuring fluid escapes easily through the nipple .... don't open it enough and wheel pistons will move.

Another problem if unbelievable amounts of air, is that air can be drawn in allong side of nipple thread, Solution is to close nipple before releasing pedal. wait a couple of seconds after pedal is up, before opening it again. OK, its a slow old job.

Some early LWB front brakes have adjustable steady posts. Come back to us if yours has.

602

When your brakes have been bled and your brake pedal travel is long on the first press and then short after, it sounds like you need to turn the adjusters on the back plate. This is caused by the shoes retracting to far, which means they have to be pushed further before they contact the drum. Hence the long pedal travel first time you press the pedal.

Hi,

Your 11" front brakes have four pistons, same as the 10". Don't be fooled by counting the cylinders. 11" pistons are smaller, so your pedal should feel better.

Fitting big cylinders on the rear will make your pedal travel further, and cause them to lock up earlier. Neither is good, so personally, I would retain the small cylinders. I have even fitted small cylinders to rear of a LWB.

It is becoming fashionable to fit 11" rear brakes to front of SWB. Get 10% better brakes, double adjusters, and still easy to bleed. And nobody wants them ..... yet.

602