I have a skytrak 9038 that has fully hydraulic brakes with a accumulator. I don't know much about these brake systems. skytrak serial number is 5A044 I checked the accumulator for pressure and there is no pressure so I see that I need to get that recharged. Problem is that when I press brake there is no stopping. I get feedback through the pedal. It almost feels like it pushes back on your foot intermittently.
You need to fix the accumulator first for sure-it will act all kinds of ways if it isn't right. You probably have another issue as well. Is it internal brakes or external caliper style? Get a gauge and if it has pressure test ports under the cab hook to the brake port and see if it shows pressure when you apply the foot brake. If not and you have internal brakes get some plugs and plug the brake lines off and test again. If you have pressure then hook lines up one at a time and see when pressure fails and there's your culprit. Does the parking brake work properly if hydraulic? Check for external oil leakage around the wheel ends where the axle shafts come out of the planetaries. It sounds like you have some pressure and the pulsing or "push back" may be the system trying to supply enough pressure to something that's leaking down. Trying to "keep up" so to speak. I'd recommend having the accumulator resealed and then recharged especially if there's no pressure and it has age on it. Just recharging may be a waste of time and money.
It has external calipers. The parking brake is cable actuated. I am going to have the accumulator resealed and recharged this week. I guess then go from there