Just bought a new to me ranger 188vs. The trailer has surge disc brakes. While driving it home, there was a jerking every time the brakes disengaged no matter how hard or soft the stop.

I took the brakes apart today and while filthy, the calipers, discs, and pads looked ok. I cleaned them up good.

There was full fluid in the reservoir.

The problem I noticed was when bleeding the system at the end. Using the screwdriver in the top hole of the hitch with the cap off, it pumped a couple times but was very slow to return and as a result I wasn't getting any good pressure in the lines.

Could this be a bad master cylinder??