Ok today the brake the brake pedal went to the floor. I got the truck home and went online to look up a new master cylinder. The truck has a Wagner Lockheed with a bolt down cap and all the ones i find online are different. Has anyone ran into this before?

 

It would help to have some info on just what truck you have. There are a lot of different applications.

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75 f250 high boy

 

Are you sure it is the M/C? Did you look under the truck to see if a wheel cylinder is leaking, or a brake line? My 76 F250 high boy does not have a bolt on cap. You have a picture?
This is what is on my 76: