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smcurro · 07-14-2007, 10:30 PM

I can't tell you how master cylinders work or how they fail, but I know for some reason they really aren't an expensive part. A replacement master cylinder should be around 25 dollars - at least that's how much mine cost me (for a 1970 cutlass, 1968 skylark and a 1979 camaro). Whenever I have to work on brakes on a car for whatever reason, I always change the master cylinder if I'm unaware of its age. It's some pretty cheap insurance and saves you one BIG headache. I don't know of any way to test a master cylinder; they're usually pretty nasty anyways so I just chuck them and paint the new one. Theres very little additional work to replace the master cylinder if you're already going to bleed the lines. Now, as for the brake booster, those are a bit more work to change but can be tested easily. The easiest way to detect a bad booster is if the pedal becomes immediately hard to push once the car is shut off. A brake booster in full working order should have vacuum leftover after the car is shut off for 1-3 full pedal extentions. The best way to definitively test if the booster is still any good is to test it with a vacuum pump. A vacuum pump is a niftly handheld pump with a gauge attached that tells you how much vacuum you are creating. A standard vacuum pump kit comes with all sorts of adapters that you will need to use to adapt to the vacuum inlet hole on the left top of the brake booster. My vacuum pump is made by a company called mityvac and I find it is a very useful and valuable tool. Adapt the pump to the brake booster and start pumping! Be sure to have a tight seal throughout when using the vacuum pump. A standard brake booster holds a good amount of air but you will see on the gauge that you are slowly building up vacuum. If after many strokes you cannot build up any vacuum or if the booster does not hold the vacuum you have built up then your brake booster is no good. You will have to buy another as I have never heard of these things being able to be fixed. The only company I've ever heard of that makes replacement boosters is a company called cardone, and their products are readily available at autozone. An interesting occurance I noticed is at autozone it is actually cheaper to buy a brake booster AND master cylinder together from cardone than a brake booster alone, but this is of course only for the cars I've worked on. The brake booster and master cylinder should run you about 90 dollars. (Again, this is how much it was for a 1970 cutlass, 1968 skylark, or 1979 camaro.) Now, you will notice the brake booster is between the master cylinder and firewall. It is indeed possible to change the booster without opening the brake lines to the master cylinder but this must be done VERY carefully. If you feel you can't do this go ahead and take off the brake lines; this is better than accidentally breaking them. The first step is to get under that dashboard and detach the connecting rod from the brake pedal. Secondly, remove the master cylinder from the brake booster. Finally, the brake booster is attached to the firewall with four studs pressed into the firewall. Be sure the studs do not spin in the firewall and chew out the metal when removing the nuts. Some time or another you should have noticed that the vacuum line to the brake booster has a check valve on it. If you remove the check valve from the hose you can test it with that handy vacuum pump. When the vacuum pump is attached to the engine side of the valve, it should hold zero vacuum and just suck through. When the vacuum pump is attached to the booster side, it should hold vacuum indefinitly and not leak any vacuum. If the check valve fails these tests then go ahead and replace it, there should be a new one provided with the new brake booster. The rest of the deinstallation/installation is pretty straightforward but don't hesitate to ask me any questions you may have. I have done this procedure three times with a 100% success rate. It sounds like you most likely need a new master cylinder and not a new brake booster, but go ahead and check the booster and the check valve if you have a vacuum pump. The booster is a lot easier to change while changing the master cylinder than by itself.

07-14-2007, 10:30 PM	#17 (permalink)
smcurro Junior Member Join Date: Jul 2007 Posts: 14	I can't tell you how master cylinders work or how they fail, but I know for some reason they really aren't an expensive part. A replacement master cylinder should be around 25 dollars - at least that's how much mine cost me (for a 1970 cutlass, 1968 skylark and a 1979 camaro). Whenever I have to work on brakes on a car for whatever reason, I always change the master cylinder if I'm unaware of its age. It's some pretty cheap insurance and saves you one BIG headache. I don't know of any way to test a master cylinder; they're usually pretty nasty anyways so I just chuck them and paint the new one. Theres very little additional work to replace the master cylinder if you're already going to bleed the lines. Now, as for the brake booster, those are a bit more work to change but can be tested easily. The easiest way to detect a bad booster is if the pedal becomes immediately hard to push once the car is shut off. A brake booster in full working order should have vacuum leftover after the car is shut off for 1-3 full pedal extentions. The best way to definitively test if the booster is still any good is to test it with a vacuum pump. A vacuum pump is a niftly handheld pump with a gauge attached that tells you how much vacuum you are creating. A standard vacuum pump kit comes with all sorts of adapters that you will need to use to adapt to the vacuum inlet hole on the left top of the brake booster. My vacuum pump is made by a company called mityvac and I find it is a very useful and valuable tool. Adapt the pump to the brake booster and start pumping! Be sure to have a tight seal throughout when using the vacuum pump. A standard brake booster holds a good amount of air but you will see on the gauge that you are slowly building up vacuum. If after many strokes you cannot build up any vacuum or if the booster does not hold the vacuum you have built up then your brake booster is no good. You will have to buy another as I have never heard of these things being able to be fixed. The only company I've ever heard of that makes replacement boosters is a company called cardone, and their products are readily available at autozone. An interesting occurance I noticed is at autozone it is actually cheaper to buy a brake booster AND master cylinder together from cardone than a brake booster alone, but this is of course only for the cars I've worked on. The brake booster and master cylinder should run you about 90 dollars. (Again, this is how much it was for a 1970 cutlass, 1968 skylark, or 1979 camaro.) Now, you will notice the brake booster is between the master cylinder and firewall. It is indeed possible to change the booster without opening the brake lines to the master cylinder but this must be done VERY carefully. If you feel you can't do this go ahead and take off the brake lines; this is better than accidentally breaking them. The first step is to get under that dashboard and detach the connecting rod from the brake pedal. Secondly, remove the master cylinder from the brake booster. Finally, the brake booster is attached to the firewall with four studs pressed into the firewall. Be sure the studs do not spin in the firewall and chew out the metal when removing the nuts. Some time or another you should have noticed that the vacuum line to the brake booster has a check valve on it. If you remove the check valve from the hose you can test it with that handy vacuum pump. When the vacuum pump is attached to the engine side of the valve, it should hold zero vacuum and just suck through. When the vacuum pump is attached to the booster side, it should hold vacuum indefinitly and not leak any vacuum. If the check valve fails these tests then go ahead and replace it, there should be a new one provided with the new brake booster. The rest of the deinstallation/installation is pretty straightforward but don't hesitate to ask me any questions you may have. I have done this procedure three times with a 100% success rate. It sounds like you most likely need a new master cylinder and not a new brake booster, but go ahead and check the booster and the check valve if you have a vacuum pump. The booster is a lot easier to change while changing the master cylinder than by itself.