pressure differential valve leaking brake fluid

[gringo]

on my 86 f250 can't keep brake fluid in master cylinder for one whole day without it emptying out. pressure differential valve is where its leaking. not sure how to replace it or best next step to take

 

[bigwhitebus]

on my 86 f250 can't keep brake fluid in master cylinder for one whole day without it emptying out. pressure differential valve is where its leaking. not sure how to replace it or best next step to take

lmc truck part only place i have found new ones. or dealer