Spongy brakes - no leaks, suspected bad master cylinder
#16
Rennlist Member
Thread Starter
I haven't gotten that far on the Subaru yet - a timing belt, water pump, rich condition, messed up VVT, and misfires in high vacuum have all popped up. It's a 1-owner 2010 XT with 116k and full dealership service history /rant But it does need routine brake service soon.
Going back to the 944, I forgot about that check valve on the booster. On bimmers they're integral to the venturi (or suction jet pump, as they call them). I remember getting a hiss when I unhook the vacuum line going to the "reservoir" by the battery tray a few hours after turning off the car - would this be a sufficient test of that valve? It's been a few months so I'll need to check again. To me, that would indicate that air can only flow out of the booster circuit.
Going back to the 944, I forgot about that check valve on the booster. On bimmers they're integral to the venturi (or suction jet pump, as they call them). I remember getting a hiss when I unhook the vacuum line going to the "reservoir" by the battery tray a few hours after turning off the car - would this be a sufficient test of that valve? It's been a few months so I'll need to check again. To me, that would indicate that air can only flow out of the booster circuit.
#18
Rainman
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Going back to the 944, I forgot about that check valve on the booster. On bimmers they're integral to the venturi (or suction jet pump, as they call them). I remember getting a hiss when I unhook the vacuum line going to the "reservoir" by the battery tray a few hours after turning off the car - would this be a sufficient test of that valve? It's been a few months so I'll need to check again. To me, that would indicate that air can only flow out of the booster circuit.
if your booster is original you got your money's worth for that diaphragm.
#19
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Thread Starter
Yes I did, I made 100% sure everything is lined up. I used the crank cog marker (I believe it's lined up when all 4 pistons are at 50% stroke). Replaced all rollers and the water pump / thermostat. Made me realize how easy 944 belt jobs are The VVT angle and misfire issues were present before, on the original timing belt. I checked before I removed it, and it was also in time. I didn't rely on locking and reusing the positions of the camshafts when I did the job, I re-aligned them to make sure I was starting fresh. No CELs or anything. Thread 1, Thread 2, Thread 3
It really chaps my ***, because I can't even really drive the 944 while the Subaru gets figured out because of the brakes but, it'll all get sorted sooner or later.
It really chaps my ***, because I can't even really drive the 944 while the Subaru gets figured out because of the brakes but, it'll all get sorted sooner or later.
#21
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Yes I did, I made 100% sure everything is lined up. I used the crank cog marker (I believe it's lined up when all 4 pistons are at 50% stroke). Replaced all rollers and the water pump / thermostat. Made me realize how easy 944 belt jobs are The VVT angle and misfire issues were present before, on the original timing belt. I checked before I removed it, and it was also in time. I didn't rely on locking and reusing the positions of the camshafts when I did the job, I re-aligned them to make sure I was starting fresh. No CELs or anything. Thread 1, Thread 2, Thread 3
It really chaps my ***, because I can't even really drive the 944 while the Subaru gets figured out because of the brakes but, it'll all get sorted sooner or later.
It really chaps my ***, because I can't even really drive the 944 while the Subaru gets figured out because of the brakes but, it'll all get sorted sooner or later.
#22
Drifting
Your car detects a foreign object with the Volvo calipers....remove the infection))
Subaru stuff...
My opinion, but subaru uses a dash more phosphates in their branded oil to keep the oil control valves and sensors clean pertaining to the cam timing. Try subaru oil and filters.
Valve covers, the spark plug seals and the cover fasteners are vacuum leaks. At 100k, they're baked and probably pooling oil at the plugs. 1/4" stuff works fine but you may need to guess the torque on at least one fastener.
I assume a manual but some automatics have a 'lifetime' spin on ATF filter under the fender liner.
Subaru love but the piston clearance is super tight for the expansion characteristics of the assembly. Weepy head gasket coating allow the slow poisoning of the oil with coolant which fouls the bearing halves. The motor only has to get hot once, and it's over. Dropping a JDM mill is actually cheaper than doing the head gaskets in the long run.
Subaru stuff...
My opinion, but subaru uses a dash more phosphates in their branded oil to keep the oil control valves and sensors clean pertaining to the cam timing. Try subaru oil and filters.
Valve covers, the spark plug seals and the cover fasteners are vacuum leaks. At 100k, they're baked and probably pooling oil at the plugs. 1/4" stuff works fine but you may need to guess the torque on at least one fastener.
I assume a manual but some automatics have a 'lifetime' spin on ATF filter under the fender liner.
Subaru love but the piston clearance is super tight for the expansion characteristics of the assembly. Weepy head gasket coating allow the slow poisoning of the oil with coolant which fouls the bearing halves. The motor only has to get hot once, and it's over. Dropping a JDM mill is actually cheaper than doing the head gaskets in the long run.
#23
Rainman
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Had to do that on my EJ25 DOHC engine. Pulling the covers sure made swapping spark plugs cake, too.
#24
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Thread Starter
Ok gents, I did 3 tests on the booster.
1) Pump pedal with engine off. It gets stiffer every time, eventually only the pedal freeplay is left. Check.
2) With the pedal still pumped up from the first test, start car. Pedal should sink when engine catches. Check.
3) After idling with no brake manipulation, depress the pedal and kill the motor. Pedal shouldn't rise back up.
...mine pops up like nothing happened. Now I'm getting somewhere. My first thought is the check valve - but after the car was off for an hour or so, I pulled the vacuum line going between the check valve and the vacuum 'reservoir' by the battery tray. It hissed and was clearly under vacuum.
That indicates that the check valve between the booster and reservoir is working. But shouldn't there be another check valve between the booster and the rest of the vacuum system? Is it built into the venturi? Maybe the booster is just bad. I would hate to replace it when it could be a simple fitting though.
Subaru: I run Mobil 1 0W-40 European car formula oil and Idemitsu ATF-HP fluid, fwiw. All OEM filters. I got new iridium plugs to put in, and I also read about undoing the motor mounts and jacking it up. That's especially helpful on the turbo models apparently. I'll get there, but the 944 is my main squeeze and I have to get it stopping right first....
1) Pump pedal with engine off. It gets stiffer every time, eventually only the pedal freeplay is left. Check.
2) With the pedal still pumped up from the first test, start car. Pedal should sink when engine catches. Check.
3) After idling with no brake manipulation, depress the pedal and kill the motor. Pedal shouldn't rise back up.
...mine pops up like nothing happened. Now I'm getting somewhere. My first thought is the check valve - but after the car was off for an hour or so, I pulled the vacuum line going between the check valve and the vacuum 'reservoir' by the battery tray. It hissed and was clearly under vacuum.
That indicates that the check valve between the booster and reservoir is working. But shouldn't there be another check valve between the booster and the rest of the vacuum system? Is it built into the venturi? Maybe the booster is just bad. I would hate to replace it when it could be a simple fitting though.
Subaru: I run Mobil 1 0W-40 European car formula oil and Idemitsu ATF-HP fluid, fwiw. All OEM filters. I got new iridium plugs to put in, and I also read about undoing the motor mounts and jacking it up. That's especially helpful on the turbo models apparently. I'll get there, but the 944 is my main squeeze and I have to get it stopping right first....
#26
Rainman
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there is nothing in the venturi aside from an air passageway and a ~1/8"
orifice drilled in a little aluminum thing clamped inside.
no check valve aside from whatever might be inside the little valve thing that plugs into the booster itself.
the vacuum can might be under vacuum just fine but the booster could still be leaking around the diaphragm. it's open to atmospheric pressure on the firewall side, on the engine side is the vacuum chamber and the differential times the diaphragm area makes your pedal assist.
orifice drilled in a little aluminum thing clamped inside.
no check valve aside from whatever might be inside the little valve thing that plugs into the booster itself.
the vacuum can might be under vacuum just fine but the booster could still be leaking around the diaphragm. it's open to atmospheric pressure on the firewall side, on the engine side is the vacuum chamber and the differential times the diaphragm area makes your pedal assist.
Ok gents, I did 3 tests on the booster.
1) Pump pedal with engine off. It gets stiffer every time, eventually only the pedal freeplay is left. Check.
2) With the pedal still pumped up from the first test, start car. Pedal should sink when engine catches. Check.
3) After idling with no brake manipulation, depress the pedal and kill the motor. Pedal shouldn't rise back up.
...mine pops up like nothing happened. Now I'm getting somewhere. My first thought is the check valve - but after the car was off for an hour or so, I pulled the vacuum line going between the check valve and the vacuum 'reservoir' by the battery tray. It hissed and was clearly under vacuum.
That indicates that the check valve between the booster and reservoir is working. But shouldn't there be another check valve between the booster and the rest of the vacuum system? Is it built into the venturi? Maybe the booster is just bad. I would hate to replace it when it could be a simple fitting though.
Subaru: I run Mobil 1 0W-40 European car formula oil and Idemitsu ATF-HP fluid, fwiw. All OEM filters. I got new iridium plugs to put in, and I also read about undoing the motor mounts and jacking it up. That's especially helpful on the turbo models apparently. I'll get there, but the 944 is my main squeeze and I have to get it stopping right first....
1) Pump pedal with engine off. It gets stiffer every time, eventually only the pedal freeplay is left. Check.
2) With the pedal still pumped up from the first test, start car. Pedal should sink when engine catches. Check.
3) After idling with no brake manipulation, depress the pedal and kill the motor. Pedal shouldn't rise back up.
...mine pops up like nothing happened. Now I'm getting somewhere. My first thought is the check valve - but after the car was off for an hour or so, I pulled the vacuum line going between the check valve and the vacuum 'reservoir' by the battery tray. It hissed and was clearly under vacuum.
That indicates that the check valve between the booster and reservoir is working. But shouldn't there be another check valve between the booster and the rest of the vacuum system? Is it built into the venturi? Maybe the booster is just bad. I would hate to replace it when it could be a simple fitting though.
Subaru: I run Mobil 1 0W-40 European car formula oil and Idemitsu ATF-HP fluid, fwiw. All OEM filters. I got new iridium plugs to put in, and I also read about undoing the motor mounts and jacking it up. That's especially helpful on the turbo models apparently. I'll get there, but the 944 is my main squeeze and I have to get it stopping right first....
#27
Rennlist Member
Thread Starter
Thanks for the info. Planning on borrowing a mity-vac tomorrow. Clarification on the booster leaking around the diaphragm. When I do the "#1 test" above that involves pumping up the brakes with the car off - wouldn't the booster fail to build up pressure if it was leaking? The pedal being stiff except for free play after a few pumps seems to indicate that there's no leaking.
#28
Drifting
More testing...
Can you pull the lines and cap off the feeds from the master to eliminate what's upstream and down stream?
Are your flex lines in decent shape? Internal swelling would explain the symptoms.
(My tribeca brakes have always been soft, for 100k miles, but it makes modulation easier on snow and ice.)
Can you pull the lines and cap off the feeds from the master to eliminate what's upstream and down stream?
Are your flex lines in decent shape? Internal swelling would explain the symptoms.
(My tribeca brakes have always been soft, for 100k miles, but it makes modulation easier on snow and ice.)
#29
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More testing...
Can you pull the lines and cap off the feeds from the master to eliminate what's upstream and down stream?
Are your flex lines in decent shape? Internal swelling would explain the symptoms.
(My tribeca brakes have always been soft, for 100k miles, but it makes modulation easier on snow and ice.)
Can you pull the lines and cap off the feeds from the master to eliminate what's upstream and down stream?
Are your flex lines in decent shape? Internal swelling would explain the symptoms.
(My tribeca brakes have always been soft, for 100k miles, but it makes modulation easier on snow and ice.)
#30
Rennlist Member
Thread Starter
Edit: I videotaped my firewall under hard pedal presses and it didn't move.
Got a MityVac and adapted it to the brake booster. I followed these tests:
http://ssbrakes.com/attachment/84298...0Diagnosis.pdf
In short, it DOES leak, but it seems to be very slight. When I put 15" of vacuum on it (20 would have taken all day with this tiny pump), it drops about 1" every 5 minutes. I repeated this at 5" vacuum and the leak rate was the same.
When I pump it back to 15" and press the brakes, I watched the gauge drop roughly 5" like it should. Pressing it again did the same thing. After the 3rd press, the MityVac read atmospheric.
Finally, I pumped it back up to 15" and held down the brake pedal for 30 seconds. It initially dropped to around 11-12" but then continued to slowly drop. I would estimate that for every 30 seconds I depressed the pedal, it fell by about 1".
So that means it's leaking. But the real question is, would this magnitude of leakage completely wreck power brake assist? It's obviously holding vacuum to some degree, it doesn't quickly drop. I'm having a hard time attributing the poor braking to a slight vacuum leak like this. At idle there should be around around 15-20" of vacuum acting on the booster or whatever is created by the venturi, if that vacuum is higher.
To get a reference, I also tested my 951's booster. It held zero vacuum it was in a fire, so I'm guessing it got destroyed.
Got a MityVac and adapted it to the brake booster. I followed these tests:
http://ssbrakes.com/attachment/84298...0Diagnosis.pdf
In short, it DOES leak, but it seems to be very slight. When I put 15" of vacuum on it (20 would have taken all day with this tiny pump), it drops about 1" every 5 minutes. I repeated this at 5" vacuum and the leak rate was the same.
When I pump it back to 15" and press the brakes, I watched the gauge drop roughly 5" like it should. Pressing it again did the same thing. After the 3rd press, the MityVac read atmospheric.
Finally, I pumped it back up to 15" and held down the brake pedal for 30 seconds. It initially dropped to around 11-12" but then continued to slowly drop. I would estimate that for every 30 seconds I depressed the pedal, it fell by about 1".
So that means it's leaking. But the real question is, would this magnitude of leakage completely wreck power brake assist? It's obviously holding vacuum to some degree, it doesn't quickly drop. I'm having a hard time attributing the poor braking to a slight vacuum leak like this. At idle there should be around around 15-20" of vacuum acting on the booster or whatever is created by the venturi, if that vacuum is higher.
To get a reference, I also tested my 951's booster. It held zero vacuum it was in a fire, so I'm guessing it got destroyed.