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I just fitted a new MC and I bench-bled it first, but I couldn’t help wondering – *why* do we bench-bleed in a vice? Why not just install the MC on the car, then fit the bleeder tubes to route the fluid back into the reservoir, and use the brake pedal to pump the piston until there are no more bubbles? Am I missing something here?
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I’m curious about this too, I just inadvertently drained mine doing a caliper replacement.
Maybe it takes less time cause the pedal doesn’t really do much? (after a brake job how many pumps till you even have them touching the rotors again?)
The only thing I can think of is that bleeding it on the car will make it a 2-person job, since one person has to be watching the fluid. If you bench-bleed, one person can operate the piston *and* watch the fluid. But other than that, I can’t see what’s different about clamping the MC in a vice compared to installing it on the car.
Brake fluid is a world class paint stripper and bench bleeding keeps some of the possible mess away from the vehicle.
A second reason is to get air out of the master cylinder without pushing it down the brake pipes. Once in the brake pipes air has to move all the way to the wheel for removal.
I’ve fooled with master cylinders where I was still getting bubbles of air out after 15 minutes of stroking. So, there’s a third reason, it is more convenient in a vice.
[quote=”barneyb” post=116679]
A second reason is to get air out of the master cylinder without pushing it down the brake pipes[/quote]But I’m talking about using the bleeder tubes that typically come with a new MC. The ones that screw into the output ports on the MC and then route the fluid directly back into the reservoir and stop any air being sucked back into the MC when you release the pedal. If you bled the MC on the car with those tubes installed, it would be almost no different to doing it in a vice, but with the advantage that you’d be using your foot to push the piston.
As I mentioned above, it would make it a 2-person job, but I reckon it would be easier and faster than bench-bleeding.
I have had people do just that then end up with no brake pedal and no end of problems. They then bring it to a shop (me) and I install a new Master Cylinder after bench bleeding and have zero issues.
Most MC have in the instructions to bench bleed first
When bench bleeding you can feel when the MC is properly bled which is something that cannot be done with a foot and pedal.
But what’s actually different about bench-bleeding? As far as I can see, the only mechanical differences are that the MC is attached to the car rather than held in a vice, and you’re pushing the piston using your foot rather than your hands.
Otherwise, the process is identical – push the piston, the fluid comes up the bleeder tubes and back into the reservoir, and air can’t be sucked back into the MC because the tubes are submerged in fluid. Repeat until no more bubbles.
Doing it that way makes it a 2-person job for sure, while bench-bleeding is a solo job, but apart from that, it’s exactly the same process. For sure most MCs say to bench-bleed first, but I genuinely can’t see any reason why it *has* to be done in a vice with your hand pushing the piston (or with something like Cardone’s syringe).
Indeed, if you use the brake pedal method when the engine is off you’re not getting any power assistance, so the only meaningful difference is that you’re using your leg muscles to push the piston, not your arm muscles.
I’ve never bench bled a master cylinder because I’ve gotten into a habit of changing and bleeding fluid throughout the entire system because as those seals brake down in the master cylinder those small particles end up somewhere else in the system which could lead to braking issues or blocked lines. I know it rarely happens but I believe its just good practice. Also I use vacuum bleeders.
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