Fading brake pedal

GMJim · Post by **GMJim** » 19 Aug 2018, 17:24

Hello all. First post and hoping for suggestions. Not that it makes much of a difference but I've been a certified mechanic for about 45 years. Mostly GM and north American cars and I've done many many brake jobs but my wife's 2008 C30 T5 is a real puzzle. I've done a lot of searching and have found nothing conclusive. I replaced both rear calipers because the right one was leaking from the parking brake lever/shaft. I was very careful not to introduce air in the line and as soon as the old caliper was unscrewed from the hose the new one was installed. Both rears were bled properly. Prior to this the brakes were fine. After starting the engine I found the brake pedal would slowly drift to the floor when applied. With the engine off i could pump the brakes up and apply a lot of pressure and the pedal would hold fine but with a light pressure it would sink. I have had this happen with north American cars and found that bleeding brakes would allow the piston in the master to move further that it normally would into an area of the bore that had a buildup of crud and would cause a seal failure. Pedal would sink but no leaks in the system. Changing the master cylinder fixed the problem every time. Knowing this I felt the problem with the C30 was the same issue. So $220.00 later with a rebuilt master cylinder (even the dealer MC's are rebuilt) I have the same problem. I'm at a complete loss to figure this one out. The ABS was/is suspect but I don't believe changing a couple of calipers and a quick brake bleed would cause it any damage? Any Volvo mechanics out there have any idea what's wrong?
Thanks

Post by **jimmy57** » 19 Aug 2018, 19:31

Have you taken it off the lift and driven it? This is going to be hard to take but the amount of force a brake needs is determined by your brain sensing deceleration and you apply force to pedal until your brain decides the vehicle is slowing at the expected rate. Once you depress brake pedal while vehicle is sitting still you lose the reference for how hard you are pushing. Your legs hold up your entire body weight without any real sense of effort so we have little "feel" for how hard we push. The amount of force you are pushing with may be swelling brake hoses and stretching calipers open. Please drive it before you do anything more.

I learned this lesson when I was a Chevy Tech in the 70's and worked on lots of corvettes.
I train techs now and hybrid brake by wire systems will blow your mind. The pedal has 75% of its travel against a spring with sensor tracking pedal travel and then electric regen retarding, and if pedal pushed more, hydraulic fluid pressure is applied via active booster and/or ABS pump. If you push pedal past 75% then you have touched the booster input pushrod (what is normally attached to pedal assy.) going into back of booster for direct brakes if the system fails. You can sit still and push the brake pedal and think you shouldn't move car as there is no brake pedl effort that builds to normal level. Put the car in gear and move off and touch pedal and the vehicle slows and stops and you'll swear suddenly the pedal got hard and feels like normal brake pedal. You didn't nothing more by moving than to calibrate your brain to foot connection.

GMJim · Post by **GMJim** » 20 Aug 2018, 07:05

Thanks for the reply and psychological assessment. Don't get me wrong, I'm open to ALL suggestions. This pedal is solid when the engine is not running (no vacuum). After pumping it a couple of times it remains at the height it stopped at for as long as I hold it down. When there is vacuum to the booster (engine running or vacuum pump) the pedal sinks with light effort but not with hard effort. It feels like a bleeder is open or there is a CONSTANT leak, but NO leak. Prior to doing the initial work I had no reason to try a panic stop to lock up all four wheels so I don't know if braking power has been affected but it will not lock up the wheels now. It does stop very well despite the issue I'm explaining but just short of locking up. I've found this common on most of the late model cars I've worked on. This issue appears to be common because while searching the internet to find a solution I have come across several examples of the same problem described exactly the same??? The car stops and I'm confident driving it BUT not convinced there isn't a problem. Some people have claimed that vacuum bleeding solved the problem. All I have is a pressure bleeder that I used at 10 PSI and did not see any air.

GMJim · Post by **GMJim** » 20 Aug 2018, 15:19

You know I have to admit, I have never thought about this quite this way so I went out to my truck, a 2006 Silverado and tried the same test with the brake pedal. It does the same thing! I would have never guessed I would get the same result. Thanks for the explanation jimmy57! Ya learn stuff every day!
GMJim

Post by **oragex** » 20 Aug 2018, 16:44

But have you bleed the master after installing it? (check youtube videos with the guys to keep the pedal depressed then release the line nuts). Air can also get into the ABS valve body.

GMJim · Post by **GMJim** » 20 Aug 2018, 16:53

Got it covered thanks.

Post by **jimmy57** » 20 Aug 2018, 17:16

I have given that same explanation dozens or maybe hundreds of times to very frustrated techs calling me about what to do after trying so many things. I got a great sense of them thinking I'm so full of crap while I'm telling them to go drive it. And then they call me back and say they knew that wouldn't work but they needed to take it off their lift to make some money and the damned thing had a normal pedal feel. If you want to really get a wake up then have some one start engine and depress brakes hard while you hold rubber brake hoses and feel the calipers yield. You would never get to that high pressure stopping the car. The wheels would be slipping on pavement and ABS would have been active long before you got the pedal that far down.

GMJim · Post by **GMJim** » 20 Aug 2018, 17:25

Keeping an open mind will help you more than hurt you. Like I said I've been doing this a long time and still learning. Brakes are working fine now. What's very frustrating is the lack of available information (service manuals) on Volvo's. I bought a Vida CD on line but don't find it very useful. Takes too long to load and hard to navigate and limited information. As a GM tech I have no trouble getting service manuals for GM or any north American cars easily.
Thanks again.
Jim

KLR142 · Post by **KLR142** » 24 Jan 2019, 16:09

GMJim wrote: ↑20 Aug 2018, 16:53 Got it covered thanks.

That sounds like you found a fix, did you?

GMJim · Post by **GMJim** » 24 Jan 2019, 17:05

This issue went away as the pads re seated. When the calipers were changed it caused the pads to be positioned in a slightly different location on the rotor. Pedal was mushy and felt as if it was sinking. After about 50 or 60 miles the pedal was back to normal. As far as the sinking pedal goes there is some substance to Jimmy57's theory. I tried doing the same thing in my 2006 Silverado and it exhibited the same thing? The C30 pedal is solid now.