IF.....everything is original to the Tractor, then it should have the Check (or Relief) valve as seen on the right.
The possibility exists that if the "plunger" is stuck in the down position, you would experience what you are having.
You can use some penetrate (ie: PB Blaster, Kroil, etc) and spray the plunger area. Make sure the plunger moves "freely".
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Roland Bedell
CC Models: 100, 105, 1450, 782, (2) 784, & 2072
Buy: Made in the USA 