Brake Booster Leak

Sweed · Post by **Sweed** » 04 Feb 2020, 07:55

I have a brake booster leak somewhere on my 2006 XC70. I have replaced the seal behind the pedal which I dont think was actually bad in the first place. I have never heard hissing there even when listening with a stethoscope. I cannot figure out where the leak is, but is causes the vacuum pump to run intermittently when car is on. It will run constantly when the pedal is pushed. I have been listening around all the external grommets and no sign of leaks. Are there any other known points of failure on these boosters? Can the pedal position sensor leak internally? Possibly the seal behind the master cyl? I have verified that the hose going to the booster does not leak and the pump does quit cycling when I block that hose off.

Post by **jimmy57** » 04 Feb 2020, 09:27

The booster can leak where master cylinder bolts on, pedal sensor, or inside (rear) at regulator valve where pushrod from pedal goes. Hae you pulled up on pedal with gentle force to see if elec vac pump cycling stopped?

Sweed · Post by **Sweed** » 04 Feb 2020, 14:11

I have tried that and it does not seem to make a difference. I can hear an extremely slight possible air leak near the pedal sensor. Its debatable if Its actually a leak though. This is listening with stethoscope. I may try pulling the sensor and replace the oring and replacing the master cyl seal next. I feel as though replacing the booster is not needed after doing the pedal rod seal. Would you agree? It has to be an external leak, right?

Post by **oragex** » 04 Feb 2020, 15:23

The pump may will spin for the first seconds after starting a cold engine when the pedal is depressed. Once the engine has build enough vacuum, it should get quiet. Check also the pump switch and all three hoses (especially the intake and booster ones - notice which hose goes where on the switch when placing it back (marked "motor" towards the brake booster) . Also make sure the connector is really pushed all the way in (depress the metal clamp and push it in). The switch breaks easy if you grab it by the top cap when pulling the hoses off (twist and pull on each hose to remove them). You also have the hose going into the booster that has a small seal at the booster

Sweed · Post by **Sweed** » 05 Feb 2020, 13:20

The issue is the booster not the valve or pump. When I pull the hose off the booster and put my thumb over the end to plug it up, the pump quits running completely.

Is it normal for the pump to run when the brake is depressed and the car is not running? Is it normal for the pump to have a brief cycle every 15 seconds when the engine is not running? Maybe I am trying to troubleshoot an issue that isn't there? This is my first newer Volvo so pardon my ignorance. Like most Swedish things, they are, well.......unique.

Post by **jimmy57** » 05 Feb 2020, 23:09

No, one of the common reasons a booster is sreplaced is the regulator valve the pushrod goes into doesn't close the atmospheric vent. The pedal being released is supposed to shut the vent and connect vacuum to the backside of diaphragm. When you push pedal the vacuum is shut off and then atmospheric vent opens as pedal is pushed and pushrod begins to move master cylinder. If the regulator valve (booster control valve also is a term used) doesn't close the vent then when the pedal is released you can have the vacuum for back side of diaphragm leak through the vent.

Post by **oragex** » 06 Feb 2020, 11:26

With engine off and ignition on (POS II), depressing the brake pedal should wake up the vacuum pump - that would be normal. As for the pump cycling every 15 sec after shutting the engine off and parking the car, I think it's what jimmy says above, I call it the booster seal at the pedal plunger. I had exactly that
www.youtube.com/watch?v=PRbWD2OGytk
www.youtube.com/watch?v=saX-GG6s1Ck

Sweed · Post by **Sweed** » 06 Feb 2020, 12:18

oragex wrote: ↑06 Feb 2020, 11:26 With engine off and ignition on (POS II), depressing the brake pedal should wake up the vacuum pump - that would be normal. As for the pump cycling every 15 sec after shutting the engine off and parking the car, I think it's what jimmy says above, I call it the booster seal at the pedal plunger. I had exactly that
www.youtube.com/watch?v=PRbWD2OGytk
www.youtube.com/watch?v=saX-GG6s1Ck

Yeah it's funny. That was apparently what was supposed to be 100% guaranteed the issue. I never hears the hiss from there, but I replaced it. Not the issue. I am going to replace the pedal sensor as I just discovered I have a code for it and I am definitely hearing a small leak there. I will report back with result.

Post by **oragex** » 06 Feb 2020, 12:35

Can also just spread a little bit of silicone caulk around the sensor, gives an idea at no cost

Georgeandkira · Post by **Georgeandkira** » 06 Feb 2020, 17:12

To oragex: As usual, many thanks for the vids.

These are the latest videos on the subject of the "scratch prone teflon plunger and it's seal" I've seen. I've read a bunch of posts too.

OEM parts or their suppliers must change. That's all I can conclude. I've seen 2 styles of outer protective dust cover. My car has none and there was no history of brake work.

One video had a different seal than the one in oragex's video though I'd bet there's no difference in function.
Mine is a short, 2 knuckle boot which feels like it's filled with lube. It doesn't resemble the "edgy looking" seal in either video.
My white plunger is smooth as glass AND THAT IS THE EXPOSED PART.
The seal felt perfectly supple and appeared new. With no problem, proceeding with any peeling back for inspection wasn't necessary.
Brakes in our cars are just "lower and softer" than others.

The other video showed a small tube of Volvo branded TPFE lube for the white cylinder but I bet the dielectric grease is adequate.