Bleeding or Flushing Brake and Clutch Hydraulics

This document last modified May 25, 2006.

For tips about flushing the clutch in a V70, S70 or C70 Volvo, click here.

Comparison of Bleeding Tools

There are a variety of methods to bleed brake and clutch hydraulics, and I think I've tried them all! I have a few comments about each of them:

The two-man method

This is probably the oldest method out there: get somebody to step on the pedal while you open the hydraulic line, and have them release the pedal after you close the line. Then repeat that cycle, ad infinitum, until the line is completely flushed with new fluid. Be careful about leaving the line open when your partner releases the pedal, or you'll just be sucking air into your lines, which is exactly the opposite of what you're trying to do.

There are some advantages to this tried and true method. It's cheap, it's reasonably fast, it's fairly reliable, but there are some occasional difficulties in rounding up a helper when you need one, and you're bound to get annoyed with each other. What's more, he'll drink half of your beer or, if you were foolish enough to recruit your wife or girlfriend for this effort, you'll have to buy an expensive dinner. Maybe this isn't so cheap after all.

SpeedBleeders

My pal Tosh swears by Speedbleeders, as they make his one-man pit crew that much more efficient, and he doesn't have to share his beer. These are a one-way check valve that is installed in place of the OEM bleed screw. Open it up, go to the driver's seat and pump away, all by yourself. Take your best guess about how many pumps you need to make, and then get up to close the bleed screw. Repeat for all four lines.

On the plus side, there's no bulky tool to lug around, as the bleed screws are always mounted on the car. On the downside, you're getting into and out of the car often, your legs get more exercise than you would doing an hour on a StairMaster, you can't monitor the color of the fluid coming out of the lines, and they're not portable so you can't use these on another vehicle. The price is right, at about $8 each.

Mityvac

Mityvac is a hand-held vacuum kit. Open up the bleed screw, hook up the vacuum pump along with the optional fluid catch reservoir that comes with certain versions of the kit, and start squeezing the pump trigger. About half-way through flushing one line, your hand should be sufficiently tired that you'll be glad that you don't have to do this job much more than once per year.

The Mityvac works just fine for brake bleeding, but clutch cylinders don't like this thing at all. When you apply vacuum to the clutch hydraulics, the master cylinder collapses. On some cars, like my Volvo, you can't just pull the pedal up to restore the clutch pedal -- you have to apply pressure to the hydraulic system. (See below.) Fortunately, my Mazda had no such problems, as the pedal has a solid link to the master cylinder.

Another problem is that vacuum bleeders have a tendency to allow air to get past the threads of the bleed screw and into the slave cylinder. As such, it can be difficult to get a really firm pedal, which is the goal of this effort in the first place. You can minimize this problem by keeping vacuum on the bleed screw while you slowly close it off. Hopefully, the threads will seal off outside air before the valve portion of the bleed screw shuts down the hydraulic fluid flow.

Lastly, be aware that some versions of the Mityvac come with attachments to use it as a pressure bleeder, but only while hooked up to the bleed screw. This is called reverse bleeding, and that's how I restored my clutch pedal in my Volvo when I first tried to bleed that clutch system. I should mention this about reverse bleeding: I don't think it's normally a good solution, as reverse bleeding will tend to push all of the little "stuff" that tends to accumulate in hydraulic systems -- worn O-ring "dust," rust, slave cylinder gremlin droppings, etc. -- back into the fluid reservoir. That means that all of that "stuff" is going to get back into the hydraulic systems, helping to wear down the sealing components of the system just so much quicker.

Power Bleeder

This is it! I'm convinced that the Power Bleeder is the best brake bleeding tool on the market. Hook it up to your master cylinder, with or without fluid in its tank, and pump it up to 10 to 20 psi. Then go to each corner of the car and open up the bleed screw until fresh, light-colored fluid comes out. Without exaggeration, I've been able to completely flush the brake system in my car in under 20 minutes. That's flush, not merely bleed, the system. Try that with any of these other methods!

Home Made Pressure Bleeders

I've heard of guys making one of these with a cheap garden sprayer and an old brake reservoir cap and clear hose. I'm sure that can be made to work just fine, but it'll still end up costing half of what the Power Bleeder costs, and probably won't be made as well or have a pressure gauge built in. The hour or two that it takes to buy and assemble all of the parts isn't worth the savings to me, but some folks like little projects like that. To each their own.

Clutch Pedal Problems on my Volvo S70

I remember the first time I endeavoured to flush the clutch fluid on my '99 Volvo S70. Volvo calls for this as routine maintenance, and mine was due. I figured "I've bled brakes before... the clutch should be the same principle." Wrong! I must have spent 5 or 6 hours trying to do this, expecting it to be a fifteen minute job with a vacuum bleeder. I could bleed and bleed and bleed all day long, and I did, but no matter what I did, I kept finding that the clutch pedal was to the floor and it would not return to it's starting position with pressure in the system. It had me wondering if perhaps I had damaged the slave cylinder during my attempts at fixing this. At various times during that futile effort, I was truly panicking, as this model year was the first for which Volvo incorporated the slave cylinder and throwout (release) bearing into a single assembly, and it would require dropping the tranny to fix it. That's big bucks or big labor, neither of which I wanted to expend.

After a full day of trying to deal with this "little" job, I gave up when I ran out of daylight. A little web surfing for "clutch bleeding tips" was very revealing. What I found was that many people have had the exact same problems when trying to bleed clutch hydraulics. I found web pages for Porsche, Audi, Volkswagen, Honda, Acura, Saab, Chrysler -- you name it; this is a very common problem. With a very easy solution.

Vacuum bleeders, such as Mityvac and others, remove the fluid that the system counts on to push back on the master cylinder piston, so you end up with a collapsed piston and a clutch pedal to the floor, and no pressure to return them to their starting point. The two man bleeding method often yields the same results. The only reliable method that I was able to find for bleeding clutch fluid is to use a pressure bleeder.

Volvo seems to have one additional peculiarity that I haven't found on any other clutch hydraulic system that I've ever worked on or heard about: if you use a pressure bleeder at anything less than about 20 psi -- or a vacuum bleeder at any pressure -- the master cylinder allows air into the lines. It's as though there's a spring-loaded air bleed valve within the master cylinder that's stays open at any lesser pressure, forcing more air into the system than fluid, and resulting in the collapsed pedal that gave me all the trouble that I described earlier. It was very strange, but the cause and the solution are both repeatable: use low-pressure or vacuum and you'll get no clutch pedal; use higher pressure, and you'll have a pedal. Even after flushing at higher pressure, the pedal is never where it should be (or as firm as it should be) until after about a block of driving, but at least I've found the secret to bleeding this thing. I hope this tip saves you some time!