what does FAM7821 actually do?

[rien_naalden]

Hello Mokers,

I have a problem with my braking system: valve FAM7821 was leaking, so I had a new one fitted.
After fitting the new FAM7821 in, my Main Brake Cilinder (LB06BZ078AD) went broke.
So I ordered a new MBC: GMC227 as is described everywhere as the suitable one, with the yellow band.

By the way: my Moke is a Portuguese, build in november 1988.

What happened: my rear brakes didn't work after these changes.........

The early MBC has exactly the same fittings as the new one, and yes, I checked for the right fittings.

Now I did override the FAM valve for the rear circuit, and all works well, even measured on a test bench.

What is the exact use of the FAM7821 ? As far as I van deduct, it only closes circuit A or B when this circuit is leaking....
The MBC itselfs splits the circuits and gives the right pressure for front and rear circuit.

All parts, old en new ones from AP braking systems. Can a new FAM7821 be broken?

ANYONE ?

Great thanks in advance, kind regards, Rien.

[spider]

Oh, those god aweful horrible things!

What were they thinking?

OK, because the Moke is so light in the rear compared to the front, if you were to do a sudden stop, all the weight will transfer to the front wheels and off the rears causing them to lock up and usually spin the car around quite quickly. To over-come this situation, the factory fitted a pressure shut off valve (or in a few models, an load / inerta valve).

On early cars with single circuit Brakes, they fitted a simple but very effective pressure shut off valve to the front rail of the rear subframe, like this;-

https://www.minispares.com/product/Clas ... o%20search

When they went over to tandem circuit brakes, some bright spark had it in their head that they needed to balance the shut off of the rear circuit to the front - enter the FAM 7821, only they never really worked in that way, but do shut off the pressure to the rears at a certain point to prevent the rear brakes from locking.

If you have fitted one of the 'Yellow Tag' Master Cylinders, and plumbed it the way that is recommended, then it won't work too well at all. The Yellow Tag MCs have a stepped bore in them to basically increase front Brake Pressure, however, in many countries they are illegal as it is very easy to run out of brake fluid well before the front pad wear out, this is not supposed to happen.

OK, now, what to do.

If you don't mind making up (or having made up) some brake lines, then I'd suggest doing away with the FAM 7821 (pipe through / past it) and fitting an old style Pressure shut off valve, or if you can get one, an Inerta Valve, these fitted to the side of the rear sub-frame and automatically adjusted the effectiveness of the rear brakes depending on the load in the rear, the slope of the road and the deccelleration rate. Note that the angle that they are fitted at is critical!

Another alternative is to fit the correct MC, they are available;-

https://www.minispares.com/product/Clas ... o%20search

[Nigel(no top)Sykes]

Wow ... I know now why I always go for the basic old fangled single line system

[Doug G]

I swapped the front drum brakes on my Mini Moke for disks.

Happy as Larry now (or however the saying goes )

[spider]

Wow ... I know now why I always go for the basic old fangled single line system

Yeah, they do have a lot going for them.

As many of us would know, the tandem set up - in theory - was done so that a failure of one circuit still left the other to stop with. Only trouble is, on Mokes (and Minis), all the various factory set up don't work that way. I found this out many years ago when bleeding the brakes on a tandem set up for the first time, which is effectively the same as a circuit hydraulic failure, that the pedal would go right to the floor.

They also made the system on some of the vehicles that it was fitted to, far more complex than it needed to be, like the fitting of that silly FAM 7821 Pressure Shut Off Valve!