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It's not often that I do a write up with this much excitement, but I want to share the results of an afternoon's experiment with vacuum filling a brake system. To summarize, the result is the best brake feel I've ever experienced with minimal lever travel that suddenly hits a "wall." Best part is that the process is instant!
I was inspired by a recent Ohlins shock service I did, which involved hooking up the shock to a vacuum that sucked all the air out and while the inside of the shock was under vacuum oil is injected so that there are no air bubbles in the shock. It's nearly impossible to hand bleed an Ohlins TTX shock due to all the tiny crevices inside for air bubbles to hide. That is the reason Ohlins rear shocks and gas charged front forks need to be vacuum filled.
Why not try the same with the front brake? I planned to machine a M10x1.0 adapter for the master cylinder to use as a fill port, but I came up with a simple way that any person can do with a vacuum pump and some Silly Putty.
My front brake system is RCS19 master cylinder connected via 2 separate Galfer SBK lines via double banjo to a pair of GP4-RR calipers. Vacuum bleed setup is shown below. A vacuum pump is mandatory. You cannot use a air compressor brake bleeder because it cannot draw a proper vacuum. Bleeder wrench (11mm for Brembo, 8mm for OEM) is highly recommended.
![263497]()
With all pistons pushed in (fresh rebuild), I fill the reservoir and bleed from each caliper with a vacuum until fluid shows up (do not squeeze brake lever). Point here is to get some fluid in the lines so our tiny reservoir doesn't run dry. Tighten up the caliper bleed bolts and disconnect the vacuum hose.
![263498]()
Turn the vacuum off. Now we focus on the master cylinder. Unscrew the master cylinder bleed bolt 1/2 turn and seal around the threads with some Silly Putty. This is necessary because there is a small amount of air leakage around the threads. Teflon tape won't work because it cannot seal straight threads under any significant pressure differential.
![263499]()
Take a paper towel and wrap it around the hose from reservoir to master cylinder. Then clamp it down with a pair of plastic pliers (I did not have locking ones that would've made it easier).
![263500]()
Turn the vacuum on and you will see a ton of bubbles come up the hose hooked to the master cylinder bleed bolt. The entire braking system is now placed under a vacuum. Wait until there are no air bubbles in the fluid running into the clear tubing. This takes about 2-3 minutes to fully stabilize.
![263501]()
Fully close the master cylinder bleeder then release the pliers from the reservoir hose. Fluid level in the reservoir will quickly drop (about 1/2) as the fluid replaces all the air that was sucked out by the vacuum. At this point, the pistons are still all the way in. Fill up the reservoir and pump the brake lever (flick the lever on release) until the brake pads touch the rotor.
![263502]()
That's it! If you want a really thorough job, you can repeat the process but the pistons will get sucked back in and you will have to pump them back out. I try to describe the feel in the diagram below. There are so many crevices in calipers, lines, fittings, etc. that add up to a small space where air can hide. You can bleed all day, run bottles of fluid, rebleed after riding, etc. to try and get that air out but it's not easy because there are blind spaces where air has no way to go up. That air acts as a cushion between the brake fluid and master cylinder piston so feel is diminished. It's like putting a foam pad on a table and pressing on it. Eventually it gets hard and you hit the table but there's a mushy bit and it's in that part where brake modulation occurs because long before you hit the table the front wheel will locked up.
![263503]()
Brake lever travel before it stiffens up is only 5/8 inch (16mm) measured at the TIP of a LONG lever. This is true even braking with the front wheel spinning on a stand. Granted I am testing this on Brembo brakes but I would be very curious how well this works on OEM brakes. At the very least, it's a massive time saver because it takes 5 minutes to perfectly bleed a system and requires a bare minimum amount of brake fluid.
![263504]()
One very interesting observation is that the "zip-tie" trick for stiffening up a mushy lever has NO DIFFERENCE on my vacuum filled system. For those of us who have tried it, the zip-tie on the brake lever makes the brake lever TEMPORARILY stiffer. But that is exactly how my brakes feel ALL THE TIME now.
Based on this observation, I believe (I may be wrong) that the zip-tie on the brake lever puts the brake fluid and small amount of air in the system under pressure such that the air dissolves in the fluid over several hours. That may be why the brakes feel so good after taking off that zip-tie but quickly revert as the dissolved air comes back out as bubbles.
Also, any residual air in calipers will expand as the calipers heat up. This may explain the mushy sensation that occurs when brakes are used hard on the track. A vacuum filled brake system may alleviate this issue.
I was inspired by a recent Ohlins shock service I did, which involved hooking up the shock to a vacuum that sucked all the air out and while the inside of the shock was under vacuum oil is injected so that there are no air bubbles in the shock. It's nearly impossible to hand bleed an Ohlins TTX shock due to all the tiny crevices inside for air bubbles to hide. That is the reason Ohlins rear shocks and gas charged front forks need to be vacuum filled.
Why not try the same with the front brake? I planned to machine a M10x1.0 adapter for the master cylinder to use as a fill port, but I came up with a simple way that any person can do with a vacuum pump and some Silly Putty.
My front brake system is RCS19 master cylinder connected via 2 separate Galfer SBK lines via double banjo to a pair of GP4-RR calipers. Vacuum bleed setup is shown below. A vacuum pump is mandatory. You cannot use a air compressor brake bleeder because it cannot draw a proper vacuum. Bleeder wrench (11mm for Brembo, 8mm for OEM) is highly recommended.
With all pistons pushed in (fresh rebuild), I fill the reservoir and bleed from each caliper with a vacuum until fluid shows up (do not squeeze brake lever). Point here is to get some fluid in the lines so our tiny reservoir doesn't run dry. Tighten up the caliper bleed bolts and disconnect the vacuum hose.
Turn the vacuum off. Now we focus on the master cylinder. Unscrew the master cylinder bleed bolt 1/2 turn and seal around the threads with some Silly Putty. This is necessary because there is a small amount of air leakage around the threads. Teflon tape won't work because it cannot seal straight threads under any significant pressure differential.
Take a paper towel and wrap it around the hose from reservoir to master cylinder. Then clamp it down with a pair of plastic pliers (I did not have locking ones that would've made it easier).
Turn the vacuum on and you will see a ton of bubbles come up the hose hooked to the master cylinder bleed bolt. The entire braking system is now placed under a vacuum. Wait until there are no air bubbles in the fluid running into the clear tubing. This takes about 2-3 minutes to fully stabilize.
Fully close the master cylinder bleeder then release the pliers from the reservoir hose. Fluid level in the reservoir will quickly drop (about 1/2) as the fluid replaces all the air that was sucked out by the vacuum. At this point, the pistons are still all the way in. Fill up the reservoir and pump the brake lever (flick the lever on release) until the brake pads touch the rotor.
That's it! If you want a really thorough job, you can repeat the process but the pistons will get sucked back in and you will have to pump them back out. I try to describe the feel in the diagram below. There are so many crevices in calipers, lines, fittings, etc. that add up to a small space where air can hide. You can bleed all day, run bottles of fluid, rebleed after riding, etc. to try and get that air out but it's not easy because there are blind spaces where air has no way to go up. That air acts as a cushion between the brake fluid and master cylinder piston so feel is diminished. It's like putting a foam pad on a table and pressing on it. Eventually it gets hard and you hit the table but there's a mushy bit and it's in that part where brake modulation occurs because long before you hit the table the front wheel will locked up.
Brake lever travel before it stiffens up is only 5/8 inch (16mm) measured at the TIP of a LONG lever. This is true even braking with the front wheel spinning on a stand. Granted I am testing this on Brembo brakes but I would be very curious how well this works on OEM brakes. At the very least, it's a massive time saver because it takes 5 minutes to perfectly bleed a system and requires a bare minimum amount of brake fluid.
One very interesting observation is that the "zip-tie" trick for stiffening up a mushy lever has NO DIFFERENCE on my vacuum filled system. For those of us who have tried it, the zip-tie on the brake lever makes the brake lever TEMPORARILY stiffer. But that is exactly how my brakes feel ALL THE TIME now.
Based on this observation, I believe (I may be wrong) that the zip-tie on the brake lever puts the brake fluid and small amount of air in the system under pressure such that the air dissolves in the fluid over several hours. That may be why the brakes feel so good after taking off that zip-tie but quickly revert as the dissolved air comes back out as bubbles.
Also, any residual air in calipers will expand as the calipers heat up. This may explain the mushy sensation that occurs when brakes are used hard on the track. A vacuum filled brake system may alleviate this issue.
