How to diagnose, repair, and bleed master cylinder and brakes.

[TDot]

In my search to figure out my spongy brake issue all anyone talks about is bleeding the brakes which I know is not my issue. I couldn't find anything here so I tracked down master cylinder issues and here is what Ive found if bleeding and pads don't fix your issue.

Also, with a brake booster test all you have to do is pump the brakes with the engine off and make sure it gets rock firm with no give. If it gets firm in about 3 to 5 pumps your brake booster is good....from my understanding.

Some vids might seem redundant. The reason I put all of these is each have an extra tip, clarification, or example that others don't. So it will be good to watch all. And I've realised the more vids you watch, the more confidence you'll build to do the job. Hope this helps, good luck.

https://youtu.be/4oi3Y6HMCbc (bleeding cylinder and brakes, good vid. Only thing, don't do like he did and make sure to COMPLETELY bleed cylinder. A few tiny bubbles can form a big bubble.)
https://youtu.be/7rImn6nJ4Dw (Best bench bleed example)
https://youtu.be/zoewhwUu3UQ (All ways to bleed brakes and master cylinder)
https://youtu.be/Xk2zGvKfrhc (How the master cylinder works, animation)
https://youtu.be/WYB4vOv7Iag (How the master cylinder works and rebuilding)
https://youtu.be/w80LkZrlp34 (How the master cylinder works and rebuilding...very good)
https://youtu.be/_aX-jP-uWgY (Self bleed manual example)
https://youtu.be/OUvBHnOh9ao (Bench bleed cylinder with just a syringe. This also shows really how easy it is to remove.)
https://youtu.be/fH8sB_unvgg (Bench bleed cylinder with no hose)
https://youtu.be/Mdcn1USVQ-w (Make your own pressure bleeder with gauge for $20-$30 part 1)
https://youtu.be/crL8IoSylk8 (Make your own pressure bleeder with gauge for $20-$30 part 2)
https://youtu.be/VbNhswhnvuA (Make your own pressure bleeder with a valve and how to use it)
https://youtu.be/YStj8ORkNqY (How to test the master cylinder)
https://youtu.be/8geH8ZYIMrM (How to test a master cylinder)
https://youtu.be/4FfBPv4cZ1I (How to test a master cylinder)
https://youtu.be/E0Fl_UM56T0 (How to test a master cylinder)

https://youtu.be/oi4BnNA3Ep4 - This one is just a bonus. He claims this is bleeding the cylinder, but it simply looks like bleeding the brakes...just that the hose is going back to the reservoir. He's not bleeding the cylinder because the other chamber could still have air...my perspective. To do it this way you'd have to have all the brakes funneled back to the reservoir. Either way, this seems like a cool way to bleed your brakes by yourself.

[Ilya]

Thanks for this post. Will add it to the FAQ.

[EniGmA1987]

If you remove your master cylinder to replace fluid and do bench bleeding, you must have the new cylinder installed and the engine on and running for a minute to then test your brake booster. By removing the master cylinder you break the airtight seal on the brake booster system. You must run the engine to suck the air out of the system and create the proper vacuum again once everything is reinstalled. If you test your brake booster after and it does not seem to test right, it could be the check valve keeping the system in vacuum that is bad, or a hose in the system.


I would also like to say that during my experience replacing my mater cylinder, brake lines, and calipers that bench bleeding the master cylinder is absolutely worthless. Our mounting is such a huge pain in the a** that you drip out the majority of your fluid and not only cause a huge mess but get air in the cylinder again just while you mount it in the car. It was completely fine the simply do the filling after everything was installed again and simply bleed it at that point.

One of the reasons you dont want to drip fluid out everywhere is because it is highly corrosive. If you spill a drop on your cars it will eat through your paint like it was nothing. So be careful of every drop, put microfiber towels all around under the brake cylinder mounting spot and on the floor under the car, and be very careful about filling the system.

[picoman]

I would also like to say that during my experience replacing my mater cylinder, brake lines, and calipers that bench bleeding the master cylinder is absolutely worthless. Our mounting is such a huge pain in the a** that you drip out the majority of your fluid and not only cause a huge mess but get air in the cylinder again just while you mount it in the car. It was completely fine the simply do the filling after everything was installed again and simply bleed it at that point.

THANK YOU, I get tired of reading about how you will never succeed with bleeding if you dont bench bleed it.

I have never needed to bench bleed any brake/clutch cylinders and concur with the added grief and mess involved. Clutch cylinders can seem a little hopeless while getting them bled, but adding a tiny bit of fluid before install helps to keep the seals lubricated and build pressure sooner while bleeding. Pressure/vacuum bleeders also speed up the process quite a bit when pedal pumping is not making progress.

[TDot]

Not dismissing what you said...good to know, but looking at our set up, you actually don't need to take it out to bleed it. The nozzles are readily accessible where you can just disconnect the brake lines, put in the lines to loop to the reservoir, and pump the brakes.

I'm going to have to disco the brake lines from the MC and pump the brakes to make sure the MC is good either way...from what I understand at least.

[EniGmA1987]

Not dismissing what you said...good to know, but looking at our set up, you actually don't need to take it out to bleed it. The nozzles are readily accessible where you can just disconnect the brake lines, put in the lines to loop to the reservoir, and pump the brakes.

I'm going to have to disco the brake lines from the MC and pump the brakes to make sure the MC is good either way...from what I understand at least.

Sure you can do a whole bench bleeding procedure when it is installed on the car with no lines connected. It is just doing that procedure on a bench outside the car, and then moving the cylinder to the car is pointless. Make sure the metal brake lines are easy to attach. Depending on their angle you may find two of them that need quite a bit of work to line up. They may have a tendency to want to cross thread.


You do have to completely remove the master cylinder from the car to inspect whether it is good or not. You cannot see the rear of the cylinder and check the seal around the input piston unless it is removed from the car.

[TDot]

If I remove the brake lines and plug the cylinder holes and press the brake (once I finish bleeding the air from the cylinder), if the brake is stiff won't that prove the cylinder is good and has no leaks...hence I won't need to check the seals? The first "How to test the master cylinder" link in the OP. Although he didn't actually say what the result of a good test should be, but I'm assuming my theory is correct because the pressure should be all held in the cylinder when the pedal is pressed.

Sounds right?