Ouch indeed–all that in only 55k miles? Doesn’t sound like a bad price for the repair though.
That is a very reasonable price. I think they were doing you a favor. I would have spent that much on the parts. But that is just me.
Pedal feels spongy, goes almost to floor: air in lines
Power brakes may obscure spongy feel.
Pedal doesn’t feel at all spongy, has excessive travel: probably warped rotor(s)
Stopping will normally be unsmooth (pulsations) if rotors are warped.
Pedal feels normal when brakes applied but slowly goes to floor over several seconds and…
Fluid on ground or other signs of leakage: Leak in wheel cylinders or lines.
No fluid on ground:Worn master cylinder.
Brake fluid can boil without water in it, under severe use. Car and Driver magazine crashed a new Nissan sports car they were testing on a race track a few years ago. Repeated laps at the track overheated the stock brake fluid, the brakes failed suddenly and off the track they went!
Also, every big semi (if you mean a tractor towing a semi-trailer) I’ve ever driven has pneumatic brakes, not hydraulic. Brakes can still overheat going down long hills, but it’s not from boiling fluid.
It isn’t what is causing the OP’s problem but, yes brake fluid can absorb water over time and can become a problem for sports cars and others who might drive their cars hard.
Because brake fluid does not really circulate within the brake line system, the fluid in the calipers at the business end of the brake lines gets old, gathers some moisture and gets worn out. This can cause it to boil under hard driving and braking. Most cars do not need it to be changed, but if you have a favorite car that you intend to keep long term, or a sporty thing that you like to do some occasional canyon-carving, then it is a good idea to at least bleed out the old stuff at the end of the lines.
It won’t normally become an issue but if all your daily driving is in Nebraska and you decide on a road trip to Colorado your old brake fluid might not be up to the hills.
Or like me, you can just do an entire fluid change every year, or move to DOT 4 which has a higher boiling point, or do both.
Regarding semi-truck brakes failing on hills, it isn’t because of the brake fluid because they don’t have any fluid in them, they have air brake systems. Air brakes work the opposite of fluid brakes. With a fluid system pressure on the brake pedal forces the brake pads to meet the drums or rotors to slow the vehicle. With an air brake system the air pressure keeps the pads away from the drums/rotors until the brake pedal is applied, releasing the air pressure.
This is safer for large, heavy vehicles because if an air line blows out, and air pressure is lost, the brake pads no longer are held away from the drums and are immediately applied. You may have seen long skid marks from trucks on the road and off to the side where the truck obviously came to a stop, some of these are from a blown air brake line. If you start up a semi in the morning you cannot even get the thing to move until the compressors have ran and the air pressure has built up.
Sorry for the slight highjack.
I admit my pucker factor went up by 20 after hearing the condition of my brakes. No way did I want to leave the shop without getting them repaired.
How are we supposed to know when the rear breaks aren’t working?
The main symptoms I noticed was the spongy pedal and it took longer to stop. Thank goodness for redundancy and the brakes did work.
I watched a video of replacing drum brake cylinders. A Bigger job than I would tackle.
