Help me understand Hydroboost/brake issue

[dfarr67]

Truck is a 1989 K1500 GMT 400.
Last summer I completed a brake upgrade to a 2005ish Suburban type four wheel disc system. After system was bled properly- worked very well.

- hydroboost was an unused unit off of fleabay with a larger diameter mount so I removed the existing HB/MC and installed a MC off of a 2005 K2500HD with the proper dia mount. Things were good. Stored the truck over winter and started it a couple times, went for a short run yesterday and there is a problem.

- brake pedal has sunk, brakes lights stay on, brakes a super hard- but I am still able to stop. I am asking because I came accross a Corvette comment that the HB doesn't like storage and may unstick after some usage, and I don't understand why something works then doesn't.

- There were some changes:
- changed the pump due to excessive steering effort on new gear- steering effort is acceptable now. This was the first step in troubleshooting the steering and condemning box.
- changed return HB line, the res is a divorced unit with can above it, from an Atro I thing- old return went right to lower res, changed return to go tp upper res- for nothing other to get a little circulation of oil. I don't see a restriction here.

[Alaskaracer]

Piston seals on the boost unit failed. Seen it before many times

[dfarr67]

What are these guys like to reseal?

[Kevin Johnson]

Be sure NOT to expose the seals to a solvent other than the correct brake fluid. It is amazing how much they can swell if this is done. That could easily cause a seal failure.

[dfarr67]

Are we are talking HB seals?----not MC seals right? so power steering/hydraulic oil only ?

[Kevin Johnson]

Are we are talking HB seals?----not MC seals right? so power steering/hydraulic oil only ?

Sorry about that. The power steering fluids (mineral oil based) can cause swelling of brake seals (glycol or SF based).

https://drivinglife.net/brake-fluid-in-power-steering/

My negative experiences were around using a solvent cleaner on MCs and calipers.

[dfarr67]

I prefer learing off of other peoples mistakes (doesn't always work), had a friend with newish chev go hunting and he tore a brake line which he crimped off and refilled with mineral oil- he replaced said brake line later at his shop but the oil destroyed everything- what I learned, it would be easier to deal with water contamination in the system as a top up. Don't know how he got GM to warranty that mess.

as an aside, rag joint gave up the ghost and I have a Borgeson intermediate shaft on the shelf, any concerns aside from a proper install with these units? Ran one for years with only issue being transmitted pump hiss bit have been warned off them- from a dodge guy.

[pdq67]

Rag joint here..

I always use the rag joint that has, "screen-wire", in the rubber washer because they are way tougher than just a rubber rag-joint washer.

The little bitty wire ends always prick me so 1st thing I do is take the washer to my bench grinder and smooth/remove the, "needles", from the outer ring.

I had an old '80 P/U that still had its ratty, "sceen-wire", containing old rag joint on it and upon inspection, it was good to go after all those years..

pdq67