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‘93 Serengeti, Oshkosh chassis. Bosch Hydromax master cylinder. Looking for thoughts on why it fails our 6 monthly Certificate of Fitness check. (NZ requirement). They put it on the rollers and it only comes up to 38%, needs to be 50% to pass. New pads 12 months ago, front brakes fine. Side to side balance is fine. Rotors look ok.
What else could it be ?

Bob,

Are they failing the hydromax or the function of the rear brakes???

If the rear brakes, it could be that the caliper sliders are frozen or some of the pucks inside the calipers could be frozen, not allowing the calipers to fully apply as designed. If you had issues with the hydromax, I doubt that the front brakes would have passed.

BTW, is this inspection for commercial vehicle class or private vehicle?

How exactly are they testing the brakes?
Do they put it on the rollers and then mash the brake pedal to the floor?

John,

They are failing the function of the rear. Front brakes are as good as they get, they tell me. Calipers or sticky pads are not suspect as there is only a little and acceptable imbalance between left and right. This is for a private vehicle, in NZ the inspection regime is the same.

And yes, they put it on the rollers and mash it to the floor. I try to heat the brakes on the way to the inspection.

Before suspecting something major, my first step would be to remove drums, sand them and the brake shoes and reinstall drum.

May just be that they are glazed.

Yes, assuming these are drum brakes on your Oshkosh chassis.

If disk, basically the same-- sand pads and disk.

May not be the fix, but cost is pretty low and can almost be considered preventive maintenance.

Ok thanks will try that

I would also check the rubber brake lines.. I've had the rubber brake lines deteriorate on the inside and cause spongy pedal feel on the various old cars / trucks I've had, uneven braking (when on the front), locking up of one wheel or the other. I would guess you just have the one flexible brake line to the axle. Worth a look.

Have you tried re-bleeding the rear brakes?
Has the brake fluid been flushed recently. Old brake fluid that is laden with moisture, will boil at a lower boiling point (see wet brake fluid boiling point vs dry brake fluid boiling point). If the brake fluid boils, it will introduce air into the brake system...