So I bought a master cylinder unit to replace my old one and when I go to bleed the brakes from what ive read you pump the brakes a bunch then turn the ignition on, you can hear the pump go on but it never shuts off like its supposed to. Then when I turn the key off you can hear a hissing noise like a air leak. Ive looked everywhere but cant see any leaking brake fluid any ideas? wondering if i got a bad unit? it has a year warranty . Been going through hell with this thing I first replaced the motor and accumulator and was still getting that screeching sound, now no sound but it wont hold pressure. ready to sell this thing!
Master cylinder leak help
- Thread starter LiLFreak
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Seasparrow
New member
Hope this helps
Seasparrow
New member
I also read this,
Hissing noise when brake pedal is depressed Inspection Service
Brakes are the most important safety components on your vehicle. Despite that importance, it’s easy to take them for granted, at least until your car begins to do strange things. Ordinarily, pressing your brake pedal should result in only the car slowing down or stopping. If you hear a hissing noise when the brake pedal is depressed, it may mean there’s something that needs to be repaired in the system.
How this system works:
On many cars, the master cylinder works in tandem with a brake booster. The brake booster is mounted to the firewall, generally in front of the steering wheel position, and the master cylinder attaches to it. The entire point of the brake booster is to make it easier to press the brake pedal and slow or stop the car.
Your brake booster works on vacuum pressure. There’s a diaphragm inside that is responsible for maintaining pressure when you’re not pressing the pedal. However, when you do press the pedal, a push rod moves forward within the booster and into the master cylinder. The side of the diaphragm facing the cabin vents to atmospheric pressure, while vacuum is maintained on the other side.
This then equalizes with pressure on the other side of the diaphragm, providing the boost needed to make pressing your brake pedal easier. The hissing noise you hear when the brake pedal is depressed could be nothing more than ordinary operation, or it could be a sign of a deeper problem. There are a couple of different potential issues here, some more severe than others.
Common reasons for this to happen:
Leaking Vacuum Line: Your brake booster is supplied with vacuum from the engine via a vacuum hose. It’s usually located near the firewall. If this hose has a leak, it can be heard within the cabin.
Leaking Brake Booster Diaphragm: The diaphragm that’s responsible for maintaining pressure within the brake booster can fail, particularly if the master cylinder is leaking fluid into the booster. When this happens, you’ll hear a hissing sound when depressing the brake pedal, but the pedal itself will be much, much harder.
Damaged or Missing Foam Silence: Most cars equipped with brake boosters also have a foam silencer that’s designed to help prevent you from hearing that hissing sound. If the silencer has degraded or been damaged, you’ll hear the hissing sound.
Failing Master Cylinder: If you hear a hissing sound and suspect that the problem is the diaphragm in the booster, the actual cause is likely the master cylinder beginning to fail. Signs that the master cylinder is leaking into the brake booster include low fluid in the reservoir with no visible leaks.
What to expect:
Hissing noise when brake pedal is depressed Inspection Service
Brakes are the most important safety components on your vehicle. Despite that importance, it’s easy to take them for granted, at least until your car begins to do strange things. Ordinarily, pressing your brake pedal should result in only the car slowing down or stopping. If you hear a hissing noise when the brake pedal is depressed, it may mean there’s something that needs to be repaired in the system.
How this system works:
On many cars, the master cylinder works in tandem with a brake booster. The brake booster is mounted to the firewall, generally in front of the steering wheel position, and the master cylinder attaches to it. The entire point of the brake booster is to make it easier to press the brake pedal and slow or stop the car.
Your brake booster works on vacuum pressure. There’s a diaphragm inside that is responsible for maintaining pressure when you’re not pressing the pedal. However, when you do press the pedal, a push rod moves forward within the booster and into the master cylinder. The side of the diaphragm facing the cabin vents to atmospheric pressure, while vacuum is maintained on the other side.
This then equalizes with pressure on the other side of the diaphragm, providing the boost needed to make pressing your brake pedal easier. The hissing noise you hear when the brake pedal is depressed could be nothing more than ordinary operation, or it could be a sign of a deeper problem. There are a couple of different potential issues here, some more severe than others.
Common reasons for this to happen:
Leaking Vacuum Line: Your brake booster is supplied with vacuum from the engine via a vacuum hose. It’s usually located near the firewall. If this hose has a leak, it can be heard within the cabin.
Leaking Brake Booster Diaphragm: The diaphragm that’s responsible for maintaining pressure within the brake booster can fail, particularly if the master cylinder is leaking fluid into the booster. When this happens, you’ll hear a hissing sound when depressing the brake pedal, but the pedal itself will be much, much harder.
Damaged or Missing Foam Silence: Most cars equipped with brake boosters also have a foam silencer that’s designed to help prevent you from hearing that hissing sound. If the silencer has degraded or been damaged, you’ll hear the hissing sound.
Failing Master Cylinder: If you hear a hissing sound and suspect that the problem is the diaphragm in the booster, the actual cause is likely the master cylinder beginning to fail. Signs that the master cylinder is leaking into the brake booster include low fluid in the reservoir with no visible leaks.
What to expect:
AuSeeker
Elite Member
Unfortunately the booster system you are referring too,..i.e. the vacuum booster is the old school brake booster system and no modern vehicle in the last 10 to 15 years or in some cases even older that I'm aware of is using a vacuum booster system any longer, most if not all of the booster systems now rely on a hydraulic booster system powered by a electric motor and 3rd Gens and onward T4Rs are included.
Seasparrow
New member
[MENTION=149207]AuSeeker[/MENTION]
A lot of 5th gens report brake hissing, how can that be if it is not air ??? The article I pasted was a generalization of the system.
A lot of 5th gens report brake hissing, how can that be if it is not air ??? The article I pasted was a generalization of the system.
AuSeeker
Elite Member
I can't say what the hissing is, but the vacuum brake booster are no longer used, all T4Rs from 3rd Gens onwards used a hydraulic booster powered by an electric motor which we all have heard when we start the engine and every few depressions of the brake pedal.
That being said there could be another part of the master cylinder or ABS system that has a vacuum line but not on the brake booster system, I haven't looked that closely to confirm or deny that there is or isn't a vacuum line elsewhere, but i have never read about such a vacuum line on our master cylinder/braking system.
The master cylinder you bought, is it a new one or a used one?
If used it could be defective in some way, the accumulator and or it's o-ring could be bad and not holding pressure, which is another common failure of the booster system besides the electric motor, the accumulator holds the pressure provided by the booster pump which is powered by the electric motor.
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It’s used the seller said he’s going to send another one out at first he was trying to tell me it’s the nitrogen gas inside the accumulator and it will go away when all the air is out of the system so I wanted to get some opinions from you guys first! Thanks I will try again hopefully 3rd time is a charm, I originally replaced the accumulator and pump to no avail this time I bought a replacement unit struck out but he does give a 1 year warranty so we’ll see...
earthrealm
Senior Member
the electric pump will soon fail. its not designed to work continiously, so expect it to burn out and quit working in a few weeks/months time. best bet is to return the unit. 2nd best bet is to try and find out if the leak is internal or external, its likely internal, if your fluid level remains constant
