Sounds like  pistons could be seized

I agree. Pistons probably not retracting after braking  

Me too. Gives me the shits can't get any consistency out of my breaks hydrostar with dual discs . Been serviced every year still f---cked. I've been told an up grade to stainless clippers would fix it but would like to hear from people who have tried. 

 Certainly sounds like pistons.

You've done well to get 6 years out of them.

I've got alko brakes that are 15 months old and have had issues with 2 of the  4 calipers,  first on being the slides and second one had a sized piston. Compressor couldn't even blow it out. Ended up servicing all 4 calipers which i will not do again.

 the  stuck one I finally got it out with a pilot bearing puller.

Not sure the price on trojan calipers but I'd seriously consider replacing them. Not worth buying pistons and seals when you can possibly buy new ones complete with pads and turn it into a swap out job, unless you have the stainless ones with all stainless pistons and pads

 other thing to check is that the pins aren't bent or aren't sitting parallel and bind up on the carrier. 

I have found it is easier to order a new set of four calipers complete with pads etc from the US.  Cost is about $100-$125 each US, get delivered to your door.  You might even be able to find them in Australia at a reasonable price now, but last time I ordered them they wanted $300 each in Australia.  Same for the hydrastar unit, 3 years ago it was AUD660 ex USA, AUD1200 in Australia.  You swap them out and then when you have some time salvage the old ones for spares.  You can also order new stainless pins, as I found they were the cause of some of my brakes seizing.  With new brakes there is less chance of stuff ups and also gives you more confidence when towing a long distance.

 Can't remember the brand , fitted new by Easytow as an option. Only just over 2 and a half years, not long I know, but no problems so far. Plenty of k's in that time. Just regular washing and a set of pads replaced. 

 It will be the pistons for sure. Get a seal kit and re do them. I replaced mine with a composite material. Bakerlite i think it is called. From Perth Brake Parts. I also put stainless slider pins in.  So far so good. Plenty of rubber greese. Brakes are not made to be dipped in salt water and then be left to sit around. There will always be issues. I also give them a good soak with the fresh water hose on the boat after I pull the boat out. That way they dry whilst i drive home. Also give them another good wash at home and then drive around the block manually putting the trailer brakes on. Get a bit of heat in them to help them dry. I also  turn the system off when I back the boat in so they are not activating whilst in the salt water.

 Callipers are a throw away item. 6 years good run.

To test if calipers are releasing,  jack up one side,  spin tyre and have someone pull the breakaway. They should lock up. Now put breakaway back in and spin tyre . You will know if the calipers are sticky. Do this on all tyres. 

For the guys saying just order new Callipers of another brand. not all brake calipers match up to other calipers mounts. Mounts need to match caliper bolts and need welding to suit calipers.

If buying new calipers of any brand. Pre grease before fitting if your mechanicly minded.

 Thanks all! 

Sounds like I haven't done too badly.  I think I'll go for stainless calipers and pistons. The guys at Easytow reckon it's the best way to have piece if mind. Better bring home some crays this year to keep the better half happy with boat maint costs! 

 I got a quote today. 4 stainless calipers , pistons , move the mounting brackets to suit pads bearings grease $1500.00 big dollars. 

 I am not sure why you would spend all that money on stainless calipers. From what I have seen the caliper is the only part that doesn't rust. The issues come from the piston, slider pins and discs. Replacing the slider pins and pistons seem to have done the job for me. Pretty cheap fix. Not sure if you can get stainless discs but I reckon that would really help.  And give them a big wash and dry each time you go out. Anyway thats what I have done. Did it my self...and I am the least mechanically minded....just got sick of spending more time on the trailer than the boat.

 Just to close out the hydraulic brakes discussion.

I got a quote from the guys at TowSafe for new stainless calipers and it was steep, but the reason I didn't go down that road was because the pistons were still the "Phenolic" (wtf that is) type, which I've assumed is the problem, i.e. the material swells after a while and becomes a tight fit in the caliper.  They didn't do a stainless piston option.

I found some guys in Queensland called Trailer Spares Direct that sell a direct copy of the Trojan caliper, except with a stainless piston.  It is a bolt in replacement.  The caliper itself is still only cast/zinc coated, but I'm not too worried about some discolouration.  

I installed them myself and the whole exercise for 4 brakes was about $700, new calipers and pads all round.  

Also, while I like to support local WA businesses, the service from Trailer Spares Direct was fantastic.  I ordered over the web on a Thursday night, the parts were posted Friday and with me the following Thursday, so basically a week turnaround, with good communication along the way.

Anyway, several local trips completed and we just returned from Augusta and all good.  Performance of the brkaes is just as good and no dragging or heating issues (touch wood). 

 Very little mate, very windy down there atm and we were mostly confined to the estuary. Never been down there before, lovely place. The locals are very canny when it cones to predicting  a weather window for offshore!