i used to have an old truck and had the same problem. for me the master cylinder was going bad and i had a bunch of shit in the brake lines. try bleeding the brakes before you buy anything new, if need be bleed the brakes out completely put all new brake fluid in, see how that works. if it doesn't help replace the master cylinder. or better yet take it to a brake specialist and get them to see whats wrong, maybe 15-30 bucks. and fix it yourself. good luck