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This is probably a completely newbie question, but it's my first car and I'm yet just learning.

So my master cylinder is defective, the brake pedal sinks and even though I can live with it, it has to be fixed.

A new one costs significant money (at least $150) and since this is a part that I've read rarely breaks, I've decided to try my luck on used market ... and here comes my lack of experience:

This cylinder has some sort of rubber seals, whilst this one doesn't look to have them or they're a different color and hard to see. Both cylinders have the same OEM number.

Actual question: Are these rubber seals swappable or can the mechanic just use some regular rubber shaped to fill those holes?

P.s: I do know brakes aren't a thing to cheap out on, but since an used cylnder is 1/10 of the price of a new one, I'm willing to give it a try, if it doesn't work out, going to get a new one. It's more about me wanting to know how these little things work. Thanks.

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    Warning, trying this may mean you never need to purchase a master cylinder or anything else ever... – Solar Mike Jun 28 '19 at 13:40
  • You aren't talking about the red plugs covering the holes for the fittings are you? – Dave Smith Jul 3 '19 at 14:40
  • @SolarMike Okay so I'm dumb with cars, but I'm not dumb enough to take it to a ride on the highway right after the Brake master cylinder change, I'm going to test on a stand first then take it to a free territory without need to brake unexpectedly, if it comes to it. So probably I'll still have to make some more purchases – Dennis Novac Jul 5 '19 at 7:54
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    Yeah, you take those out and that's where you brake line fittings go. you can leave them in to bench bleed it, or you can take them out and bench bleed it by running a line from these fittings back into the reservoir. Make sure you do bench bleed it. If you haven't done that before watch some videos on youtube or something – Dave Smith Jul 5 '19 at 8:20
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    @DaveSmith I do have a qualified mechanic to help me with that. Thank you, you've turned my world upside down :) – Dennis Novac Jul 5 '19 at 8:49
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You posted two links to images of master cylinders and then asked,

This cylinder has some sort of rubber seals, whilst this one doesn't look to have them or they're a different color and hard to see. Both cylinders have the same OEM number. Actual question: Are these rubber seals swappable or can the mechanic just use some regular rubber shaped to fill those holes?

It's hard to know what from the photo you're referencing as "rubber seals." It sounds like you're indicating that you think these photos are showing "seals" and those are the specific parts you're worried about replacing.

Master cylinders do indeed have very important seals - the seal that seals the piston in the cylinder. You can't see that seal under normal use (or in either of your photos), but that seal is what lets the cylinder build pressure as the piston moves in the cylinder. Generally, it's possible to remove and replace the seal, although that means finding the correct part, and many master cylinder manufacturers don't sell seals separately - only whole intact units. Replacing that seal means ensuring that the piston and cylinder are in good shape, and it's often the case that if the unit's condition is bad enough that the seal is gone from old age or wear, the cylinder bore may be scratched or corroded as well.

The photos you posted do have some other "seal like" parts in them. The first photo has red plugs in the fittings where the brake lines would be attached to the cylinder. These plugs are temporary, to keep the cylinder from becoming contaminated while in storage. They're removed when the unit is installed in the vehicle. in this photo, there's also a rubber bushing on the bottom of a post sticking downwards from the plastic fluid reservoir - that's likely there as a bushing to fit that post into a mounting point in the vehicle. It's probably not replaceable separately.

Your second photo is taken with the fluid reservoir removed. The photo is looking down at the top of the unit, at the mounting fittings where the reservoir would normally be mounted. Those two fittings allow fluid to flow between the reservoir and the master cylinder. There are rubber bushings inside those fittings (it's usually a friction fit for the male fittings on the reservoir). Those bushings may be replaceable separately, and they may even come in a rebuild kit with the internal seal included.

The photos you've posted look like the Ate master cylinder used in some smaller Volvos and Fords maybe 10 - 15 years ago. I did a quick google search and didn't see a rebuild kit but you may have better luck spending some more time looking.

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$150 really isn't that much. Yes, seals can be replaced. Most brake master cylinders can be overhauled, depending on the model kits are often available with replacement seals, springs and the like. There may be sites and videos with step by step instructions as well, again depending on the model.

The seals and replacement parts in the kit will be specific to that master cylinder, you can't just take any old pieces of rubber and stick them in. This is a job that needs to be done right unless you like the idea of a catastrophic brake failure, a high speed crash and possibly dying.

If you want to save some cash you may be able to get a reconditioned part that has been professionally overhauled. This can save a lot of money and give you something you can trust, just be sure to get one from a place with a good reputation.

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All master cylinders have rubber seals AFAIK. It's just a question of where. Many MCs have rebuild kits available; basically you replace the rubber seals which can deteriorate over time. These are obviously cheaper than a new or rebuilt MC, because you actually have to do more work.

  • There are other materials used apart from rubber, teflon, viton etc – Solar Mike Jul 3 '19 at 15:20
  • There is no exact fit rebuild kit for my Volvo S40 2007 anyway :| at least not one I've found in a week of searching – Dennis Novac Jul 5 '19 at 8:06

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