I have an on going issue with the Sensa Brake system on my boat trailer holding slight hydraulic pressure in the lines after a hard brake application.

It causes just enough pressure in the lines that you can only just turn the wheels by hand if you have the jack under the axles.

Even if you then disconnect from the vehicle and drain all the air out of the tank the wheels will still be dragging.

The only option is to crack a fitting on the hydraulic lines to let the pressure out and the callipers release.

I have had it looked at twice by one service agent who thought it was all the grease in the master cylinder/air piston area.

To me it is like there is either no free play between the master cylinder and the piston that pushes it however apparently this is not adjustable or the system that allows the fluid return to the reservoir is not allowing the fluid back to the reservoir when the brake application has finished.(Seal in backwards)??

Can any body suggest any thing before I replace the entire unit. 

Thanks,

 

If the trailer has not been used for a couple of months you should power the brakes up and work them a couple of times or better still take it for a run and get some heat into the system.  They don't like sitting around idle and the diaphram or o-ring will fail.  Diaphram symptoms as you describe.  O-ring failure results in fluid leaking from the control (pump) unit.

You hit the nail on the head. I sent the unit up to Jim at Trailer Parts Online in Pukekohe and it was brake fluid had leaked from the rear master cylinder seal and contaminated the o ring on the diaphragm causing it to intermittently stick in the bore and hold the brakes on ever so slightly.

I would defiantly recommend Jim to anybody looking to have this work carried out as he has a full test rig set up to check when finished.

I bought a Sensa Brake unit years ago and when it was first installed it leaked. I tried to get hold of Sensa Brake but found the company had been bought out by Alko. Thought I was going to have issues with warranty but when I got hold of Alko they luckily still backed my unit and told me to take it to someone (will remain nameless) for repair as they were the service agents. Did that and when I got it back it still leaked. Took it back again and when it came back it still leaked. I rang Alko and they supplied all the parts I needed and I rebuilt it myself. It has been fine for probably 10 years. It has just started leaking again so have decided to give it another birthday. Stripped it all down and went to source the parts I needed. I now find Alko has been bought out. Managed to get hold of the rep. They will not sell me the parts. Got hold of the one person who services them and he will not sell me the parts. Was told that if I had an accident he would be liable. What a load of %*(&^. I can buy any part for my car from any auto parts supplier and do it myself (He obviously just wanted the business). Well I had decided if I could not rebuild it myself I was going to renew the whole braking system for a different brand. Anyway I have now found out where I can get the parts I need locally. They are all standard off the shelf seals. Don't be sucked in if you want to do it yourself and have the knowledge. It is really not that hard. While trying to source the parts I have found that these units seem to have got a really bad name and a lot of boat places are not using them anymore. Not because they are a bad unit but because it has been impossible to get parts or servicing. I'm not going to undo all the wiring loom on my trailer to remove the unit and send it away when I can do it in place and have the knowledge. If you want to know where to get the parts and have the mechanical knowledge to give it a go get in touch and I will be happy to point you in the right direction.

I'd be surprised if they don't have something suitable in stock.