I think I'm going to take the trailer in to a local place and have them go through the whole brake system. It might cost me a couple hundred bucks, but at least I'll have some confidence that it was done right, and I can then learn how to do it myself when it's less critical (we're leaving for Smoke on the Water in a couple weeks).

And I have to say the OSO community rocks - not one comment about my obvious trailer ignorance!

If you are taking it in to spend money, you minght as well have them adjust the brakes, replace the bleeder valves and repack your bearings and replace the seals. If they are drums the adjustment is childs play. All this stuff is pretty easy to do yourself, and pretty inexpensive. I would say you really need to bleed the system if he replaced the master without bleeding anything, you can gravity bleed them or buy a pump like mentioned before for a minimal price. He probably never tried to loosen the valves (not easy to reach without jacking the trailer up and removing the tire) and if he did the whole assembly will come out pretty easily with some vice grips.
How do they know you need new wheel cylinders without even looking at it? I would be very surprised if they were bad. You may just need a brake adjustment unless they are discs in wich case you calipers slides may be frozen up. Just take them apart, replace the pads and lube the slides real good. Pads should be about 30 an axle. Seals will be 4 or 5 bucks a wheel. Sounds like they already talked you into a few hundred dollars without even touching it.

I think we've all been there!

You guys have shamed me into hauling it home and having a go at it myself, so I expect you jokers to be ready with your helpful advice!

OSO will come through for you.

Just replaced my surge brake to Electric/Hyd yesterday, what a difference. Total cost was around $700. but the trailer stops so well it's worth every $. It also is easy to change over, took me the better part of the afternoon.

definitely jack up each side of the trailer and check the bearings. about 6 months after I bought by boat I was backing it into the water and one of the tires floated away At the time I had never thought of checking the bearings now I check them a few times a year, loosing a tire half way across michigan might just ruin the weekend.

had a buddy that had that happen to him on the ramp

A bad shock in the hydraulic surge coupler and air in the brake system will cause severe banging of the surge coupler when taking off and stopping. I have a triple axle trailer with disc brakes on two axles and when I received the trailer new from the manufacture it had air in the brake system. After I blead the brakes the banging somewhat improved but it still hit pretty hard. I pulled the two shock absorbers out of the surge coupler and checked them as best I could for load required to move them. It appeared to me that they could not be effective as it took only 25 pounds of pressure to close them. I called the coupler manufacturer and they sent me new shocks that stopped most of the banging taking off and all of it stopping. You will still get some if you take off hard without tightening up the slack in the coupler. Good coupler shocks, brakes free of air and functioning, and proper driving techinque for surge brakes allow the hydraulic surge coupler to work very effective without the bad banging.