Thanks so much for all the help guys! These forums are such great resources for amateurs like me.



Following various advice I got the springs off pretty easily with a large pair of water-pump pliers and a lever. I may well make up something special as suggested above to put them back on again.

The parts seem largely OK except the cylinder which has scores and was leaking into its boot: new parts on order. The back plate, hub, drum and wheel will be blasted and painted.

I stripped down the foot-brake air valve.

There are air two chambers one of which was quite clean and the other was full of white powder. The cylinder bore seemed good but then I noticed that it was cracked so I ordered a new part.

Came across one of those strange bits of pricing. A rebuild kit which by me reckoning contains a handful of o-rings and a filter is UKP128. The bit you see in the first picture is called a foot-brake 'portion' and is UKP 230 complete with all the internal bits and the entire valve which includes the angle bracket that you see in the last picture is UKP403


It seems that the portion is by far the best buy so I ordered one of those.

Dismantled the rear cross-member and gearbox mount. What an elaborate set-up! I wonder why.

• 1 cross-member
• 2 large castings
• 4 rubber bushes
• 4 large bolts
• 4 special bolts with coachbolt style head and reduced threaded section
• 4 dropper plates

Putting the condensate tank back on. Does anyone know what the threads for the fittings in the air tanks will be?. I had assumed NP because of the GM connection but the guy from the local truck parts store reckoned BSP.

Attachment 351

Sorry, memory doesn't last that long.

I would favour BSP.

Turns out they are NP.

I bought a set of three plug taps on fleabay for $33 delivered. I probably will never use them to cut new threads but for cleaning out old ones they work very well.

Shiny new front brakes
 

A few shots of the front brakes going back together

Does anyone know what the two air brake circuits are called? One is the service brake and the other?

One is fed from the smaller of the two tanks and goes via the foot valve to the 'end' of the servo, nowhere else.

The other is fed from the larger of the two tanks and goes via the foot valve to the 'middle' of the servo. This one ALSO feeds the handbrake lever and the anti-compounding circuit for the handbrake.

What would they each be called? Just interested.

It depends on the age of the vehicle.

It might be Service and Secondary but Primary and Secondary circuits are normal descriptions, however, it isn't unusual to have two primaries as well as a secondary.

Typical Air-over-hydraulic System.

Modern vehicles have dual-circuit systems which might split the hydraulics as well as the air, thus reducing the possibility of total failure, so with one circuit failure you would still have adequate (if reduced) braking available.

The secondary circuit must achieve certain parameters - and might be applied through the 'parking' brake.

Many people overlook the handbrake as being available when the wheel brakes have failed, but it will usually be certified as a secondary brake application system.

The truck is 1980 and has dual air circuits driving a servo to dual hydraulic circuits. The handbrake is a spring brake on the rear axle that operates the wheel hydraulic cylinders mechanically.

The difference between the two air circuits is that when one fails the handbrake will lose pressure and the brakes will apply whereas when the other fails nothing much will happen apart from the buzzer in the cab. I can't quite get my head around the logic of having the handbrake apply when one system fails but not the other.