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Old 11-10-2017, 02:59 PM   #1
mechaniman
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Join Date: Aug 2009
Location: eugene, oregon
Default 2003 s40 poor braking ( calling all Volvo techs )

I've got a tough one here. Customers complaint is poor braking especially in the morning.

Initially the car came in and it had the all too common s40 issue of a very hard brake pedal after sitting for awhile, you'd have to put a ton of pressure into the brake pedal and it didn't seem like it'd stop. Brakes are much more responsive after it warms up.

Tested brake vacuum switch and found that it was stuck closed sending permanent power to vacuum pump which was burned out.

Replaced vacuum switch ( comes as a hose assembly with check valve ) and vacuum pump.
Re-tested and pump runs as it should, as soon as you turn the key to the 2nd position and builds 14inHG vacuum ( pressure holds, so no leaks ) start the engine and pump builds 18inHg, when you hit the brakes vacuum drops and pump turns on at 14inhg and goes back up to 18inHG. ( all within spec ) pump and switch are working perfect.

With engine off, i can pump the brakes to discharge all vacuum in booster, start the engine with you foot on the brake pedal and the pedal drops about an inch as soon as the booster receives vacuum. Booster test good.

As soon as engine warms up my vacuum gauge shows that booster has between 22inHG and 25inHG vacuum.

It really feels like I'm not getting power assist when cold but that whole system is functioning as it should ( or at least test good )

Cold braking still sucks, but it also feels like its lacking when warm as well.

I'm starting to run out of options and would love to see this car leave the shop fixed.

Here is a list of things I've tried with no improvement.

-New master cylinder ( along with flushing the whole system )
-New front brake pads and rotors ( it was a long shot but the old pads and rotors were heavily glazed and of poor quality )
-New Brake booster

Here is the only other clue I have for you, after everything warms up and my brakes are more responsive....but still isn't stopping the car fast enough.

When I do a panic stop it feels like the front brakes are trying to lock up ( way too soon )
and the ABS system kicks in.

I've heard that low ABS accumulator pressure can effect brake boost, but no way to test.

any ideas ?
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Old 11-10-2017, 03:27 PM   #2
aendrody
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Location: Houston
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9/10 its that line with he solenoid or the pump. The ABS computer or hydraulic unit may be failing, which is rare, but we've seen a couple of them in the past few years. No codes, but ABS will activate and lock wheels at lower speeds.
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Old 11-14-2017, 08:56 AM   #3
Janspeed
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how are the flex hoses between calipers and hardlines?
When these get older they can swell up on the inside causing all kinds of weird effects. The fluid can't pass through well enough. You would need extra strong pedal pressure to force the fluid through the hose. In this case reverse flow will also be problematic potentially resulting in binding brakes and excessive pad wear.

Sometimes this condition isn't even visible on the outside of the (perfectly nice looking) flex hose so don't assume that they are OK on the inside.
The only real proof is when you disconnect the hose at the caliper and let it dangle free. It should drip plenty of fluid. If very little or no fluid drips out the hose is no bueno.
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