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I did 8 trips this past week towing a dump trailer about 30 miles each way but mostly uphill when full.
Had the trailer electric brakes set to 30% as over that they would lock up.
My 1990 F250 4wd brakes worked fine until at the end of my last load the red brake light came on and my truck had almost no brakes. Feels like about 10% of normal braking.
I had to increase the dump trailer brakes to 90% to back down the driveway and dump.
The rear brake shoes are brand new and i checked the front pads and they are about 50 ot 60 %.
The pedal does not feel weak so i don't think it is the master cylinder.
Brake fluid is clean and full.
Would it be the booster or is there anything else it could be?
If the booster is there a way to tell if it is the booster itsef or the vacuum line or whatever creates the vacuum?
Thanks
 

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You can test the vacuum pump. There. Should be a manifold on the drivers side inner fender well. One of the lines out should have a cap, hook a vacuum gauge and start the truck.

You should have about 23" Hg, and if you press on the brake pedal it shouldn't drop below 13 IIRC and should recover in 3-5 seconds. If it fails, then hook the gauge directly to the line from the pump. If it fails, then replace the pump. You will need a puller like that used on a power steering pump to remove/install the pulley.
 
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