My wife has Hope Minis on her bike and they were working well, however after pulling the kids in the trailer the brake levers become stiff with little movement and the wheel(s) become immovable. The only way I found to alleviate it this weekend was to let some fluid out. However the rear lever is once again feeling much stiffer. Any thoughts? I will probably bleed them again. But it only happens after pulling the trailer.

If she's dragging on the brakes and/or using them more because of the extra weight, it'll cause the fluid to expand and push the pistons out. You can fix the effects of the expansion by just pushing the pistons back into the caliper; no need to let fluid out, and it'll only create its own problems down the line.

I'd suggest that you bleed the brakes (flushing all the old fluid out) with a decent DOT 5.1. You might also check that that your wife is/isn't over-using the rear brake. If the problem persists, and you can show that it is a fluid related problem, then you could just up each rotor by 20mm.

Ta, I tried pushing the pistons back in the other day without luck. This was several hours after using the bike. Can it take that long for the fluid to cool?

are they rubbing a bit and heating the fluid so it expands and pushes the pistons out a bit, if it only happens when your pulling the trailer it might be that the extra weight of the trailer is slightly twisting the frame and cause the brake to rub and heat up and bind?
or just old fluid thats absorbed some water and expanded, i had an old bike with some hayes 9s that wsa sat in the garage for 2 years and they had locked solid, when i bled them through they were fine but when i looked at the fluid it was like milk

If they've really been cooked, the seals could be stiffening, which would stop the self retraction of the pistons.

Old fluid has an almost jelly-like consistency. I've overhauled brakes which have left me amazed at how they were even working. Apart from the effects of heating/boiling, the fluid around the caliper seals and MC diaphragm will absorb moisture from the air which makes its way past the seal. This fluid will have a significantly lower boiling point – an obvious problem at the caliper – and will thicken long before the rest of the fluid. As tron has said, the heat of a binding brake might also affect the efficiency of the seals.

When was the fluid last flushed/replaced?

Fluid was replaced around 6-7 months ago. The bike isn't used much either. Might be that the brakes need an overhaul, they are from 2003 and still running the original pads (she doesn't like to brake!).

Do Hope offer full servicing on brakes?

Hope will rebuild for about £40 per brake, I believe. This includes pads, so it's fairly good value. A seal kit (caliper and lever) will set you back about £7. Here's a photo guide for what is a pretty simple task.

i had the same problem with my old minis, even a bleed didnt fully sort it, but new seals and they are good as new again

sounds to me like sticky pistons made worse by braking harder with the trailer.

Clean and lube. Simple to do. Pads out, caliper off, pump brake while watching pistons – if one moves much easier hold it in until both pistons are part way out. Lube with red rubber grease or brake fluid, work the pistons in and out a few times

i found with mine that to get the pistons back in its Much easier to use a pair of needle nose pliers through the top of the caliper and a normal set wide enough through the bottom, equal pressure and they slid back in,
ive towed my littlun around int he trailer a fair bit and not had issues with the pads binding, but then i'm most likely half your good lady's weight again and run big rotors (203/180), can also help to bend the pad springs out a fair bit too to help push them back,
the mech at the lbs suggests as above by pumping the piston out a bit, cleaning what you can see and squiring in some lube or ptfe/silicon spray and pushing the pistons back in and repeating until they are both free and smooth, seems to help quite a bit,

another thought could be that the caliper isnt quite true witht he disc and is sitting a little scew?? had that in the early days of fitting mine when i didnt have a scooby what i was doing! 😉

pads wise, ive still got half the pad life left i recon on my original hope pads! and theyre '04, and had a fair bit of use! gone through quite a few pairs since mind hahah!