Brake getting hard to push

Yes. The glaring clue is not the blown-up cap. It is the rubber bladder seal under the cap. You will find it is almost impossible to reset it back into the cap.

This is caused by the brake fluid being contaminated with petroleum product. All the rubber parts in the hydraulic system will be grown the same way. The lip seals inside the master cylinder will grow past the fluid return ports and block them. That keeps the trapped fluid under pressure and unable to return to the reservoir, and that keeps the brakes partially applied. The excessive heat buildup from driving the truck like that makes the brake fluid get real hot and expand. Since the fluid is trapped, when it expands, it applies the brakes harder. Eventually the truck will barely move and one or two wheels will get real hot.

The only proper fix is to remove everything with rubber parts, flush and dry the steel lines, then install new parts. That includes replacing the calipers and wheel cylinders, all rubber flex hoses, the combination valve, and the master cylinder. If any part, such as the calipers, are left on the truck, the contamination will leach out of the seals and recontaminate the new fluid.

Every week we read about someone who accidentally put power steering fluid, engine oil, or transmission fluid in the master cylinder but there are other ways to contaminate the fluid. One problem that was more common years ago was repacking front wheel bearings, then wiping hands on a rag, then reseating the bladder seal into the cap when refilling the master cylinder. The grease residue on fingertips is enough to contaminate the system.

Another problem showed up many years ago at a mass merchandiser. A new mechanic wiped out a funnel used for engine oil, then used it to fill a brake pressure bleeder ball. The residue in the funnel contaminated five gallons of brake fluid and many cars.

Experienced brake mechanics will even wash their hands with soap and water before touching any seal or other rubber parts they are working on. Also, they will never use penetrating oil to free up rusted fittings in the brake system.

Brake fluid can be contaminated with water too but that will not cause the rubber parts to swell. That will simply lower the boiling point from well over 400 degrees to closer to 212 degrees, and it will promote corrosion of metal parts but that is all. Moisture contamination comes from leaving the master cylinder cap off or the cap off the bottle of new fluid. Brake fluid sucks humidity out of the air, and it will absorb some over time in the truck's hydraulic system.

Your cost estimate is in the ball park. If you are dealing with a reputable shop owner and they have had additional similar complaints, they will be more likely to believe you and want to help. If yours is the only one this happened to, you have to look at it from their point of view. They cannot prove that you did not go home and put some contaminant in the brake fluid. This happens way more often that you would think. It comes down to who believes who the most and how concerned the shop is with their reputation.

It is not uncommon, when they cannot verify they are at fault, that they may offer to split the repair bill 50/50. As a general rule, given the mark-up on parts, they may not lose dollar-wise but you will still pay part of the bill. They may not charge again for labor, so that would save a pile. Where they lose is the mechanic cannot move on to other jobs.

What I would do is return to that shop and describe only the new symptoms and leave it at that for now. If you tell them someone else found fluid contamination, that proves right there that someone else is involved. That adds another variable the first shop has no control over and may make them less likely to want to get involved in any remedial service. Wait to see what they recommend, then ask if they will guarantee the service will solve all the problems. Expect to have to leave the truck for a few days.

If the shop just will not get involved in any way, consider looking for a community college with an Automotive program. We were always looking for live work to give our kids real-world experience and we had a dozen community members who would sit on a broken car until it fit what we were teaching. You may have to wait until they are teaching "brakes" class. We got parts at real good discounts, then marked them up ten percent to form a "breakage" fund in case we damaged something. Labor was billed at ten bucks per hour for what the job was listed as it should take, but be aware the students are only in the shop a few hours per day. It could easily take two or three weeks to get your truck back.