# Anyway to test the master cylinder?



## '89gli (Sep 2, 1999)

I've bleed my brakes a few times on my 93 Corrado w/ ABS and still have soft pedal. I'm using a powerbleeder and can't find any leaks in the brake system, so it leads me to believe I have a bad master cylinder.
Is there a way I can test this to see if it's bad?
Here is my procedure when bleeding the brakes, if this helps. I pump the bleeder up to 15psi and start at the passenger rear caliper, then other side, then passenger front and finally the driver front. I don't turn the key to the on position or pump the brakes at all. Could I possibly be doing it wrong? Any help is appreciated because I'm


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## Racer_X (Jul 14, 2002)

*Re: Anyway to test the master cylinder? ('89gli)*

If there's air in the ABS hydraulics module, you'll need to bleed that, too. 
I'm not really familiar with VW ABS systems, so I can't say exactly what's involved there. It often takes a scan tool to exercise the pumps and valves in the modulator/pump assembly and work any air out of those systems. 
As for testing the master cylinder, usually, if you press on the pedal and hold it for a minute or two, and it doesn't sink any further, its good. 
Also, did you ever bleed it by pumping the pedal? If so, did you push the pedal to the floor? Or only part way down? If you press the pedal to the floor while you're bleeding it, you can run the o-rings in the master cylinder over trash and corrosion in the MC bore and damage those O-rings. It won't hold pressure after that and will be spongey and weak.


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## '89gli (Sep 2, 1999)

*Re: Anyway to test the master cylinder? (Racer_X)*

Pedal slowly sinks to the floor. Never pump the brakes when I bleed them either. I have the powerbleeder so no air is getting into the resevoir.
Looks like I'll be taking it to a shop.


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## ATPTourFan (Jul 14, 2003)

> As for testing the master cylinder, usually, if you press on the pedal and hold it for a minute or two, and it doesn't sink any further, its good.


To clarify, this is with engine off, right? So you could lightly pump the pedal with engine off until it becomes stiff, then if you rest your foot on the pedal, it should not move even after several minutes of constant pressure.

I ask because with my engine off, after replacing a rear passenger caliper, I have a firm pedal with engine off, but the pedal is soft when engine running. I can still stop the car and lock up the brakes, but the travel is more than I've ever experienced.

My plan was to bleed all four corners (I only did the rear passenger where I was working before) in the sequence, rear-pass, rear-driver, front-pass, front-driver.

I'm assuming there's still a few little air bubbles in there. It was getting dark as I was bleeding the caliper after the replacement the other night.

Thx for the advice!


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## ATPTourFan (Jul 14, 2003)

What shouldn't really be a surprise, after a very liberal bleeding in the proper sequence (all 4 corners), my brake feels normal again. 



ATPTourFan said:


> To clarify, this is with engine off, right? So you could lightly pump the pedal with engine off until it becomes stiff, then if you rest your foot on the pedal, it should not move even after several minutes of constant pressure.
> 
> I ask because with my engine off, after replacing a rear passenger caliper, I have a firm pedal with engine off, but the pedal is soft when engine running. I can still stop the car and lock up the brakes, but the travel is more than I've ever experienced.
> 
> ...


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## Eric D (Feb 16, 1999)

'89gli said:


> I've bleed my brakes a few times on my 93 Corrado w/ ABS and still have soft pedal. I'm using a powerbleeder and can't find any leaks in the brake system, so it leads me to believe I have a bad master cylinder.Is there a way I can test this to see if it's bad?
> Here is my procedure when bleeding the brakes, if this helps. I pump the bleeder up to 15psi and start at the passenger rear caliper, then other side, then passenger front and finally the driver front. I don't turn the key to the on position or pump the brakes at all. Could I possibly be doing it wrong? Any help is appreciated because I'm



Your Corrado came with a Teves MkII ABS system and has no booster at all.
It relies solely on the modulator (this is your master cylinder), pump and accumulator to provide the power assist.
The pump can pressurize the system up to 210 bar (3045 psi). 


*In your Corrado Bentley manual you can read all about the ABS system.*

From the Corrado Bentley:

With your pressure bleeder, you start at the front left caliper.
Work with an assistant and have him/her at the caliper ready to open the bleeder.
You inside the vehicle depress the brake pedal.
Assistant opens bleeder till no air bubbles come out.


Repeat the same for the front right caliper.

The rear brakes are bled the same as the fronts.

The info from the Mk2 Golf / Jetta Bentley is different even thou the ABS system in nearly identical, why are they not identical (no EDL for an early Mk2).

*From the Mk2 Bentley:*

*4.3 Bleeding Brakes (ABS only)*

In general, the same concerns about brake fluid and brake bleeding which apply to the conventional dual-diagonal braking system also apply to ABS-equipped cars. The ABS, however, requires some additional safety precautions and slightly different procedures. 
The ABS operates at very high pressure-as much as 210 bar (3045 psi)-and is self-pressurizing. This pressure may be retained in the system even though the car is not running and has not been driven recently. Before beginning any brake work on an ABS-equipped car, the system must be depressurized. With the engine not running, depress the brake pedal 20 to 35 times to release the stored pressure from the accumulator. The reduction in pressure should be accompanied by a noticeable increase in pedal effort. 

*WARNING-* 
The procedure for releasing stored accumulator pressure must be followed prior to bleeding the front *or* the rear brakes. 

 
Once the stored pressure is bled from the system, the front brakes can be bled by conventional pressure bleeding or manual bleeding. See *4.2 Bleeding Brakes*. Since the front brakes are operated by two completely independent hydraulic circuits, the order in which they are bled is unimportant. 
The bleeding procedure for the rear brakes takes advantage of the ability of the ABS to pressurize itself. The hydraulic pump acts as a pressure bleeder. Bleed each of the rear brakes by depressing the brake pedal slightly, turning on the ignition (to run the hydraulic pump), and opening the bleeder valve. Allow the fluid to flow until it is clear and contains no air bubbles. Then close the bleeder valve and turn the ignition off. Repeat the procedure for the other rear brake. Finally, turn on the ignition and let the pump run until it shuts off. 

*CAUTION-* 
The hydraulic pump should not be allowed to run continuously for more than 120 seconds (2 minutes). If it runs longer, allow it to cool for at least 10 minutes before continuing the procedure. 

 

*CAUTION-* 
Check the fluid level in the brake fluid reservoir frequently. To prevent air getting into the system, the level should not be allowed to fall below the minimum mark indicated on the reservoir. 

 
*NOTE-* 
When bleeding the rear brakes, the actuator lever for the rear brakes' load-sensing pressure regulator must be held toward the axle beam (toward the rear of the car) to allow maximum fluid flow.


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