When my brakes get hot they are so mushy I can't even get the ABS to kick in. When I first start to drive I can get the ABS to kick in immediately and the car stops on a dime. I have replaced rotors, pads, master cylinder and fluid. Both rear calipers were hard to remove because they had no slack/space between the pads and the rotors (as if the brake pedal was slightly depressed). After I replaced the the rear rotors and pads and pumped up the brake pedal, the rotors and pads had enough slack to move the rotors by hand. The e-brake operates as it should, none of the piston boots are torn and the pistons seem to move as they should, there are no fluid leaks, the fluid bleeds as it should and I bed the new Hawk HPS pads to the new rotors according to Hawks recommendation. What is the best test for caliper problems and is that what I should be looking at? Any help is greatly appreciated. (I did search the threads so don't flame me).
Brakes Real Mushy When Hot - OK When Cool?
- Thread starter pgilliam1
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- 15 May 2004
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Seems like you have done quite a lot of things to sort it out. As far as I understand you only have problems with hot brakes and as soon as they cool down again they're working perfectly, right? If this is the case it can't be boiling fluid as it would mushy permanently afterwards even in a cold scenario.
Four hints:
1. Does any of the brake discs get hot while driving on a highway? Mine get around 40 degrees Celsius if I run with 80 mph, use mainly the gears to slow down and measure the temp.
2. You still have air in the system.
3. The brake hoses (rubber) are bad (expanding) under high temp.
4. If you have a leak in the system the level of the brake fluid reservoir will sink over time.
I can't speak for the HPS pads as I have the HP+ ones which don't fade even after several hard decelerations.
Four hints:
1. Does any of the brake discs get hot while driving on a highway? Mine get around 40 degrees Celsius if I run with 80 mph, use mainly the gears to slow down and measure the temp.
2. You still have air in the system.
3. The brake hoses (rubber) are bad (expanding) under high temp.
4. If you have a leak in the system the level of the brake fluid reservoir will sink over time.
I can't speak for the HPS pads as I have the HP+ ones which don't fade even after several hard decelerations.
Last edited:
pgilliam1, the solution is to mount a bbk, I have the ap-racing with 330mm discs and Dot5
SPECTACULAR braking.
the bad thing is the investment in money
SPECTACULAR braking.
the bad thing is the investment in money
Sounds like your pistons in the rear calipers may not be retracting fully. Might be worth taking them fully apart to inspect/rebuild them. Used to have the same problem on Yamaha rear brake calipers. The pistons would move out but not retract enough and drag causing excess heat to build up. The fix was to take the calipers apart and clean all the crud that builds up in there(more than you think even after a good fluid flush). Inspect the bores for pitting/corrosion. Replace the o-rings if all looked good and re-assemble. Bleed the system.
I haven't heard this before. What's the cause?
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If the fluid has been building even small air bubbles in the system the bubbles only get smaller with lower temperature but the pedal will still feel mushy, a little bit less but still mushy. The only way to get wrid of them is to flush until they're out of the lines.
But where does the air come from (and how does that relate to hot brakes)? I know overheating causes the fluid to boil, but these bubbles will condense back to liquid when the system cools.
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That's the information I have. :wink: I was told they remain in the system until you flush the system.
goldNSX, I tried your highway run and my brakes remained cool (it's 60 degrees/F in St. Louis).
I took the wheels off and found tiny pieces of the zinc plating from the new rotors spread over the calipers. I don't know if the plating pieces got embedded in the pads but the pads looked OK. I scuffed the surface of the pads, removed all the zinc glittered everywhere and reassembled. I bled the lines and found a couple of tiny air bubbles in the left rear caliper (tiny means the size of a pinhead - I wonder if rotating the pistons back into the caliper caused the bubbles to dislodge from inside), went for a drive and rebed the pads. Tomorrow I hope to heat em up and hope the mushy feel is gone.
Thanks to all for your inputs. Happy Motoring!
I took the wheels off and found tiny pieces of the zinc plating from the new rotors spread over the calipers. I don't know if the plating pieces got embedded in the pads but the pads looked OK. I scuffed the surface of the pads, removed all the zinc glittered everywhere and reassembled. I bled the lines and found a couple of tiny air bubbles in the left rear caliper (tiny means the size of a pinhead - I wonder if rotating the pistons back into the caliper caused the bubbles to dislodge from inside), went for a drive and rebed the pads. Tomorrow I hope to heat em up and hope the mushy feel is gone.
Thanks to all for your inputs. Happy Motoring!
How common/rare is this and can you inspect the hoses and identify this problem?
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It's not very common on the NSX as rubber parts are of very high quality. It's not very easy to detect this problem. I'd inspect the surface of the hoses and squeeze them a little bit to feel any inconsistancy. Brake hoses have a tiny hose inside surrounded by a think protective hose. If the inner hose starts to leak/fails it's not easy to detect it inspecting the visible outer hose. It may feel like a bulge egg.
There is also a date stamped on the hose like 11/91 which means they have been built in nov. 1991.
The zinc plating looks a little bit suspicious.
The last time I flushed my brakes, at one point I let the level get too low in the master cylinder and it ingested a small air bubble. I immediately caught my mistake and noted the mushiness. At one rear caliper I flushed out some bubbles that were smaller than champagne bubbles. There were not many of them, but it took about another 12 oz of fluid to fully them flush out.
So, the rubber hoses (even flexible braided stainless steel lines have a rubber core), etc, might be suspect, but even one small bubble that didn't get forced out the first time you refilled the system can raise havoc.
Sounds like it is time the check to be sure the caliper pins are not frozen. Remember these pins need to be greased and must move freely.
In your original post you mention being on the brake and the rear rotors still turn. That sounds like the pins are stuck, just a thought
.
HTH,
LarryB
In your original post you mention being on the brake and the rear rotors still turn. That sounds like the pins are stuck, just a thought
.HTH,
LarryB
Nigel and Larry -
Are the "pins" where the 2 bolts hold the calipers on to the caliper housing with the flex rubber boots around them? Mine do move in and out - but they're a little stiff (some stiffer than others). Also, can those pins be worn out? When I depress the brake pedal my passenger rear caliper has a slight twisting movement that my drivers side doesn't have and the movement has got to come from where the caliper attaches to its' housing because the housing is solid as a rock. Is it hard to take the pins off the caliper (I've never done it before)?
Thanks - and Happy Motoring!
Are the "pins" where the 2 bolts hold the calipers on to the caliper housing with the flex rubber boots around them? Mine do move in and out - but they're a little stiff (some stiffer than others). Also, can those pins be worn out? When I depress the brake pedal my passenger rear caliper has a slight twisting movement that my drivers side doesn't have and the movement has got to come from where the caliper attaches to its' housing because the housing is solid as a rock. Is it hard to take the pins off the caliper (I've never done it before)?
Thanks - and Happy Motoring!
