I have had the braking power decrease several times after UPD and the front bumper impacts the ground fairly hard. I have the brake lever recessed enough so that when a front bumper strike occurs the lever is not affected as shown below.
When an impact occurs the braking force decreases, instead of engaging after a ¼inch of lever distance it’s takes ¾inch and the braking affect is less. Some times to a point where I can’t hard lock the brake and stop the rotor completely. I need to take out my Allen wrenches and adjust the brake lever via set screw inwards so the brakes engage where I expect them to.
After some time I have another impact which reverts the engage position. The brake engages immediately and I need to withdraw the set screw out to give it my expected ¼inch travel before it engages.
Anyone else have this occur? Normally I wouldn’t worry about this but I am riding my freewheel exclusively. I am getting into longer and steeper descents so any braking loss is problematic.
I had to take my 36er to my LBS to get the hydraulic disc brakes serviced about a month ago since there was a leak in the line somewhere and it introduced air into the system. Brake lever would be fully depressed and I could still move the wheel with little force. I recall for about a month or two beforehand the brake feel started feeling “funny”.
A strange occurrence happened when I was demonstrating the faulty brakes to a relative… All of a sudden it decided it wanted to work great again 
It wasn’t until I laid the unicycle on its side for a couple minutes that it went back to being spongy.
That’s just a bit of an anecdote from me, I can’t tell whether that’s the problem in your case. Though if you’re running a hydraulic disc brake and the system hasn’t been bled in a while, it could be worth looking at.
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I don‘t know what‘s happening with your brake.
I do have a brake on one of my unis with a squashed brake line (probably happened during shipment). Also, I suspect there’s air inside the system - have already ordered a bleeding kit. Anyway, if I rapidly and repeatedly pull the lever the engagement point moves considerably. Could be worth to try this next time your brake behaves strangely; maybe things return back to normal.
When the feel of your brake lever feels unconsistent there’s most likely bubbles in the system, meaning you should bleed your brake. You can pump it very quick, if that makes it lock sooner, there is air. If it feels spongy, there’s air. Bleeding isn’t expensive, and it should be done once a year to keep the system clean and the brake working for a long time.
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