I have a John Deere 355d lawn tractor. Every time I engage blade or release brakes the mower dies out. Checked safety seat switch and its not that.

Oftentimes there are switches at the blade engagement lever and under the seat...for safety reasons. I can't start my mower if the blade drive is engaged and /or I'm not in the seat. That's a clue to check , however I'd check the starter solenoid first.

Visit the on-line store, know as reliable-store, they have the repair manuals. I'm sure you will get the solution of your problem.
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Check the plug under the seat. In fact unhook the connector and reconnect it.

Sounds like a safety switch, you just have to hunt it down, possibly in the bottom of the seat

check the seat switch first.

You have a problem with a safety interlock. I'm betting it's the operator presence safety seat switch. To check if it is the seat switch:

1. Unplug the seat switch.
2. Jumper the seat switch wiring harness plug receptacles.
3. Start your tractor & release the brake. If the tractor stays running, replace the seat switch.

you have a safty switch problem you have 4 to 6 switchs the easest way to test them ,startin with the seat get a ohms metter they are open or noramly closed

it sounds like the safety switch under seat is bad

The seat switch is doing it's job,you can;t get off the seat unless the pta is off and the tractor has the park brake engaged.Replacing the switch won't change anything.

CK THE SEAT SWICH OR THE BRAKE SWICH ONE OFEM IS BAD