Hello good people
I will try and give the correct information here, but don't 'bank' on it. I am no technician but do understand, thereabouts, how things work!
The J71 and J72 and both similar in operation, the brake is mounted on the back of the gearbox and 'locks' the propshaft. On the J71, I believe the reservoir etc., plus two switches are mounted underneath the RV and to the right hand side of the gearbox and not very accessible. One of the switches, the green one, does play up from time to time (it is known as the RGS, rotten green switch) and if you have a J71 brake that is giving trouble, this may be the cause. Also, the yellow pull switch on the dashboard on the J71 is connected to a cable and this cable can become a little stretched, hence why it has been mentioned it may need adjustment.
I have scrabbled about under my RV and there is no reservoir, switches, etc., underneath for the auto apply brake. My manual explains the J72 auto apply brake mechanisms, the actuator, pump, switches, ECM (electronic control module) etc., are mounted to the side of the radiator (under the bonnet, or hood if you like), drivers side. This has some coloured switches too. The UK Workhorse agents (Signature at Wolverhampton) reckon one of my switches is faulty, and it is this switch that is bringing the 'auto brake fail' and 'alarm' on because if it was the other switch, the brake would not work. The brake works fine either with the gearbox put into park, or the yellow dashboard knob pulled on. The 'fail' and 'alarm' stop when in park and come on when in any gear. Damn annoying. Oh yes, there is definitely no fluid leak from the reservoir. And one other thing to mention, at the back of the yellow dashboard knob, there are two wires connected to it, not a cable.
I am booked into Signature next month for a repair and only hope the 'fail' warning and 'alarm' don't drive me mad on the 120 mile journey!! Fingers crossed then.
Richard (Isle of Man)
PS - I hope you don't mind me quoting below three remarks from US websites, all relating to J71 or J72 brakes
(i)Workhorse service centre narrowed 'auto brake fail' and 'alarm' to electrical, replaced a circuit breaker that affected the 5 amp fuse circuit, didn't know why! (I suspect this was a J72 brake)
(ii) Cable adjustment. Cable that connects autopark hydraulic cylinder to brake mechanism, cable had 'stretched' causing autopark alarm. Used adjustment (under motorhome) to 'tighten' the cable, now no alarm. (I suspect this was J71 brake)
(iii) Brake locked on miles from home, removed relay which reset engine electronics, lasted 50 to 100 miles. Did it a few times, but got home ok. (I suspect this was a J72 brake)