Ford 6.0L Power Stroke Diesel :: Brakes Normal Under Light Braking But Weak During Fast Stop

My truck is a 2004 F350 6.0 as with most of the ford diesels from this era it has hydro boot brakes.

Under normal braking the truck felt fine and the brakes behaved as they always have. However if you where to depress the brakes hard, as if making an emergency stop, the brakes would not lock up in the way they did when I bought the truck two years and 80,000km ago. The problem was slow to appear and at first I blamed it on the larger tiers I was running (stock 2012 F350 wheels and tires (yes the offset is wrong)) but after switching back to my stock wheels for the winter I knew that something was not right. It was something I could live with, it did not make the tuck unsafe it was just that it did not stop as well as I knew it could and this avowed me. I replaced the front and rear calipers and when that had no effect I swapped out the master cylinder this still made little difference.

I drove the company truck all of last month (2008 GMC 2500 Duramax) and when I got back in mine I realised that I had to chase down the problem. I did a lot of reading online and after thoroughly flushing and bleeding my brake system I narrowed my problem down to either the hydro booster or the power steering pump. The plan we came up with for diagnosing was:

1. Thoroughly inspect for PS/Hydro boost and brake fluid leaks following the procedure found here Hydro-Boost Power Assist Systems: Operation Diagnosis and Repair
1a. Consider bleeding the brakes as a bit of trapped air would cause your symptoms.
1.b Consider flushing the PS system. Some suggest this works with tough steering.

2. Have a shop check the PS pressure is within spec. (assuming this is cheaper than buying the tool or just replacing the pump) If it is low, replace the PS pump.

3. If PS pump is fine, or is replaced and the problem is still there, consider replacing the Hydro booster.

It is not super complete but it was a place to start. We checked for leaks and found nothing. The pedal would not sink under braking but would say firm meaning the brake system was free of leaks.

I began a rudimentary test of power steering pump. With the truck in the drive way I could easily turn the steering wheel however, applying the breaks about half way would make turning the wheel imposable. This led me to believe the power steering pump was starting to go. The next step would be to have the pump tested using a power steering pressure gage or take the truck to a shop to have it tested. I found the pressure gage to be more expensive then the new pump so went that way. With 350,000km on the truck I figured it was not a bad thing to do even if it turned out to not be the cause of the problem.

It is a bit tricky to get the power steering pump out but after pulling the radiator line under the truck I could drop the pump out the bottom. The new pump went back in the same way and after a full flush and bleed of the system it was time for a test. What a difference! The truck stops like the day I got it. If your symptoms are like mine I would recommend you check the power steering pump. If it is the problem it is a lot cheaper then replacing a master cylinder or calipers.