have a triple axle trailer with 2 axles that have drum brakes (surge brakes) when i stop fairly hard the trailer slams and when i take off from a light even very slow i pull it 4inches hen it jerks hard then starts moving. whats the deal brakes not working?

Brakes might need to be bled or the spring behind the master cylinder might be shot. If everything is working properly it should not bottom the actuator out at every stop light.

is it hard to replace the spring

4 inches seems like a lot which could be that the brakes need bleeding and or adjusting or replacing. Mine will move maybe 2" and after a hard stop it will jerk the tongue forward before trailer actually moves.

Check your fluid level first. Could be empty and if it is, that usually means you need to bleed them which is easy with two people.

Drums are high maint on two axles. Just the slightest bit of wear and you need to adjust to keep from banging. Adjust them up snug and bleed to get all the air out of the system. They should not bang. Disc brakes don't have to adjust and are way less hassle.

4 inch's has to be empty

well how big is your trailer? do you have ellectric brakes or hydrolic brakes?

I'll have to agree here. I'd be willing to bet the master cylinder is pretty much bone dry. Refill the system, bleed it (and check for leaks), adjust the brakes back up and the banging should go away.

Ann I take off I feel mine "catch" and when I come to a stop I also feel the trailer when the tongue slides forward. Not sure I would call it like a bang but I can feel what it's doing. Just had mine completely serviced by our local truck and trailer shop to. I thought this was somewhat normal????

"Some" movement is normal. That's just the way surge brakes work. When you slow down or stop, the trailer surges forward - which actuates the brakes. How much does it surge forward? When the brakes start to take effect, the brakes in the trailer lessen the surge - which lessens the braking effect. Actaully, it's a pretty cool system and if maintained properly, it works great.

In my experience, if you're feeling a lot of movement, either your brake shoes are not adjusted correctly or you have air in the lines. Since air is compressible and liquids are not, you get excess movement.