Another weird brake problem

viphoto

viphoto

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Looked in the archives and nothing quite matches...So this started a couple of months ago and I got busy and haven't figured out whats going on... I get int he car when cold and the brakes act normal..pedal effort normal, pedal travel normal. After driving and parking the car if I get back within 15 minutes all is normal but after that the pedal effort increases substantially and the travel to full braking decreases the car stops fine but you really have to step on it. And just to make it weirder if the car cools for a couple of hours things are back to normal....I am assuming it has something to do with heat build up after shutting it off but not sure what component is causing it at this point. Any ideas or input greatly appreciated. (Weber 38's on a 3.5 if it matters)
 
When was the last time you fully bled your brakes replacing all fluid? How old are your flexible brake hoses at each caliper?
 
IT HAS TO BE THE BOOSTER
as that is the only control for the braking pressure the bleeding
will be deeper pedal movement
 
Thanks guys
In answer to the following:
"When was the last time you fully bled your brakes replacing all fluid? How old are your flexible brake hoses at each caliper?"

Have not done full brake flush but that is something I will be doing. Not sure thats where the problem is as none of the brakes are binding when the problem surfaces, although I guess if the brake fluid was boiling at at some point in one of the lines ...but again the substantial increase in pedal effort may nullify that.

"hmmm....good one. I suspect booster/servo or check valve at the very least. "

I have checked the check valve and it tests ok. Have done the brake booster check . Turned the engine off and continued holding the pedal for another 30 seconds and brake pedal stays. Then also with engine off did the
Pump the brakes five to six times then started the engine, and pedal drops like it should. Albeit both tests done while cold. Today I will try them again hot and see the results.

IT HAS TO BE THE BOOSTER as that is the only control for the braking pressure the bleeding will be deeper pedal movement

I pretty much agree but the normal tests (at least cold) don't confirm. I just don't get why it only gets weird after sitting hot....As long as I am driving at normal operating temperature it's fine...it only malfunctions on a restart after sitting hot for 20-30 min continues to malfunction when driving.

Any Additional input appreciated.

Today's Troubleshoot To Do List
Run booster tests when hot...Do the brake cleaner spray test for vacuum leaks when hot....check all lines for proximity to heat source ...replace check valve (just in case)....open a Newcastle

Stay tuned....Cheers
 
i would recommend that you replace the flexible brake hoses at each caliper if there is any doubt in how old they are - BMW at a minimum, but i would go with the stainless lines. old brake lines can expand, reducing the hydraulic pressure reaching the caliper ... therefore reducing brake effectiveness.
 
Thoughts for you..

After warming up the Master may be seizing....
The proper test to eliminate the booster is to test the booster when it's warm and not working properly with a handheld vacuum pump. Kits include all the adapters needed. Sold at stores and the web.
Undo the hose before the valve.
If there is no whooshing sound it's important !
Use the pump to see if booster holds vacuum.
Run vacuum to 20" with handheld vacuum pump
Does it hold?
Step on pedal- does vacuum decrease?
Does pedal drop?
 


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sfdon said:
Thoughts for you..

After warming up the Master may be seizing....
The proper test to eliminate the booster is to test the booster when it's warm and not working properly with a handheld vacuum pump. Kits include all the adapters needed. Sold at stores and the web.
Undo the hose before the valve.
If there is no whooshing sound it's important !
Use the pump to see if booster holds vacuum.
Run vacuum to 20" with handheld vacuum pump
Does it hold?
Step on pedal- does vacuum decrease?
Does pedal drop?
Click to expand...

Thanks Don
Took the car on an extended run this morning and noticed the pedal got harder as the car got hotter, after some stop and go and temp went to its highest the pedal effort went way up and the travel decreased. Got home and pulled hose off the valve on the engine side no whooshing sound (wrong side duh)....put it back together ran it for a couple min and took the hose off at the valve on the booster side....no whoosh either. Plugged the hose on the engine side (so no vacuum to booster) and drove the car a couple feet and the pedal feels the same as when it acts up after hot...will try again this evening to see if it feels the same when cold and no vacuum to verify its not the master seizing under heat. Looking like the booster... will pick up the tool to verify ....the vacuum pump test kit is one tool I don't have yet and I thought I had everything...
 
Another thing to check - your rotor temps
If the shaft to the Master is not releasing all the way to point of releasing the pressure in the Master then the pedal will go higher and brakes will heat up and not work worth a damn eventually seizing up.
Check your rotor temps and remember blue rotors are a bad thing.
 
Started just now cold...every thing back to normal...big whooosh when pulling the hose this time after a very short drive.... . Also no blueing on the rotors. Crazy...I just ordered a tester and will test when hot..
 
sfdon said:
Did you want to borrow a checkvalve?
Click to expand...
Thanks Don but I found one I had purchased a while ago..(not as cool looking as the original though). I installed it tonight and will further test this week...

And Bluecoupe ... No loss of brake fluid and no white smoke... and no sign of fluid being sucked through.... Thanks for the input...
 
OK, great intel, then I am thinking caliper hoses, how old are they? Have a tendency to swell internally and prevent flow. Perhaps an easy fix? Except for the brake fluid bleeding and drip.....Best of luck. :)
 
If the brake hoses are bad they can work like a one way valve. The rubber inside the hoses is collapsing and closing the tube. You can get brakes because of the pressure of the master pushing the fluid but the bad hoses can keep the pistons in the caliper from returning and then the pads drag and everything gets hot. Just because the rotors aren't blue doesn't mean this is not happening. When it does happen you can jack up the front wheels and see if they spin easily. Could be a rears as well.
 
m5bb said:
If the brake hoses are bad they can work like a one way valve. The rubber inside the hoses is collapsing and closing the tube. You can get brakes because of the pressure of the master pushing the fluid but the bad hoses can keep the pistons in the caliper from returning and then the pads drag and everything gets hot. Just because the rotors aren't blue doesn't mean this is not happening. When it does happen you can jack up the front wheels and see if they spin easily. Could be a rears as well.
Click to expand...

Thanks Gary
I haven't jacked the car up when it happens but I have pushed the car back and forth in neutral and it seems to roll free...but I will jack it up and give them a spin to verify. The plan was to change hoses if I had to crack the system open...which may be sooner than later.
 
Got side tracked....my damn 540 alternator died...(its water cooled so PITA to change...) I did install a new check valve in the coupe but only drove around the neighborhood so I haven't got it hot enough to check to see if it makes a difference. Will update soon...got the vacuum tester friday but haven't opened the box... I will update soon
 
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