Garages and Carports - electric opener problem

Hello, I have a used chain drive opener and door. When lowering the door the brake/solenoid keeps engaging/disengaging, sounds like a machine gun all the way down.
It does this only when the chain is attached and the weight of the door is applied, it does not do it when the chain/weight is off.
I did not receive any type of door closing safety devise but the door has a rubber tube that sits inside the seal that I assume is part of some type of pressure switch.
How can I bypass the safety switch?
What type of safety devise do I have /need?
Or do I need to wind the spring another turn in order to take more weight off the opener?
The only name I see on the opener is Lineal DSCO model JMB-211FD.
The control box and remote are lift-master brand.

Is the opener mounted on the side of the door with a chain that drives the torsion tube (jack shaft) or is it a trolley type mounted above and centered on the door with an arm that attaches to the door?
Your safety device is a pneumatic air hose plugged on one end and attached to a switch on other end, which will send a puff of air to switch to reverse the door when it strikes an object .the switch should have wires coming out of it leading to the control box

Hello, and thanks for your reply.The motor is mounted on the side and it drives a chain that drives the large sprocket bolted to the end of the door. I am ignorant on the terminology, but the opener drives a chain that turns a large sprocket. The large sprocket is centered on the end of the roll- up door. I should have told you it is a 18' wide X 13' tall roll-up door.
Apparently they kept the safety switch, all I have is a six inch piece of two strand wire that they cut and left attached to the motor control.
What could be causing the brake solenoid to be rapidly triggered on and off as the door is closing? It worked smoothly when I operated the opener with the chain off.

I have never seen that before and I'm not sure why it would do that, maybe the brake solenoid is going bad but that wouldn't explain why it works when chain is disconnected from the door. The solenoid gets power from a relay or the main contactor inside the control box. Some tap power from inside the motor It all depends on the manufacture and model. The brake is designed for heavy duty doors (like yours) to stop them from coasting closed or open. Short term you can mechanicly wire the brake open and disconect the wires. You should also run a new wire to the safety switch.

I will wind the spring another turn and see if that helps.