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Posted by Paul (in NY) on Monday, March 30, 2009 at 23:21:33 :
In Reply to: OT Soft Brakes in a Ford F350 posted by Ed Tavernier on Monday, March 30, 2009 at 22:43:21 :
Since you have a diesel, I am basing my answer on the probability you also have a hydroboost brake booster, powered by the power steering pump. Hydroboost Brake Boosters, can not tolerate any back pressure on the low pressure return line to the power steering pump. Back pressure will change the balance of the internal valving and in extreme cases will cause the brake to self apply when the engine is started, with the pedal going to the floor.
Try this test;
Remove the low pressure hose at the Hydroboost and plug the removed hose. Then run a temporary new hose from the hydroboost, directly onto the fill port on the power steering pump tank. Start the engine and see if all works well now, If this solves the problem, you know you have some type of blockage in the original low pressure return hose. You can also remove the low pressure hose nipple on the hydroboost and check if the .125" passage is blocked with junk. If all this proves OK, you most likely have a bad hydroboost unit.
Paul
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