Write-Up: Brake Master Cylinder - Install & Bench Bleed

Bad master for you both.

A bad booster will fail to assist (like the car is off) or have a vacuum leak that affects idle.

Brake Master Cylinder Write-Up

Tools Needed:
14mm open ended wrench
5/8" deep well socket
ratchet with long extension
brake fluid x3 (i used DOT 4 synthetic)
New/reman master cylinder (part # 25632708 for acdelco dealer replacement)
bench bleeder kit
pliers and flat head



1. Start by removing the strut tower bar if your car is equipped with one.
2. Next siphon out the old fluid from the reservoir on the MC.
I laid out a towel on the fender and below the MC because brake fluid likes to eat paint!
Note the coke bottle to catch the old fluid.


3. Un-clip the level sensor and set it out of the way. Be careful as there is a pull tab on the bottom holding the clip on.
4.Remove both nuts holding the MC on the brake booster with the 5/8" socket.
5. Disconnect the brake lines going to the MC with the 14mm wrench.
Fluid will still drain at this point so you may want to have a small catch ready.


6.Using the pliers and screw driver remove the ABS tube from the MC and set aside. DO NOT drain the fluid from this tube! If air enters the EBTCM you will need a tech-II to bleed that part of the system.
7.Finally remove the old MC from the car. Again, the brake fluid will eat paint so watch for drips.


8.Bench bleed the new MC on a vise and reinstall. I do not have a vise so i bolted it to the booster and pumped the brake with the MC disconnected from the lines.
Pump the fluid until there are no air bubbles coming through the lines.
You will need about a bottle and a half to completely fill the MC and reservoir. Only depress the piston inside the MC about 1".


9.Reconnect the brake lines and have someone hold the brake while you tighten the line nuts down.
10. Plug in the level sensor.
11. Start vehicle and check for leaks while pumping the brake.
Note, the peddle will likely be very spongy due to air in the lines. You will need to bleed all 4 wheels and top off the fluid as you go.
12. Stand back and admire your fully functioning brakes and clean reservoir.


If you liked this write-up please give me a "+" or feel free to post if i left something out.

-Matthew

Very nice write-up. Just one small correction: #3 - "Un-clip the Low-Fluid-sensor connector and set it out of the way", not the the ABS connector...

Another thing you may mention is, when bench bleeding, DO NOT depress the master cylinder piston all the way, or you may damage the internal seals.

dynomite job mr.Riv. dynomite!

fixed, and thanks for the comments.

Hey guys,

I've had similar symptoms to both 95ParkAveUltra and Matthew above. Brake pedal is spongy but will stop the car. At a full stop, the pedal can be depressed to the floor. When pumped, it acts correctly. No hard stopping power at all though.

I replaced my master cylinder today and rear two calipers. All calipers are now new. Bled and changed fluid. However, I still have the same symptoms.

Am I missing something? Is my new MC bad? Please help

- Drew

Ok I get it now.

Look for one or more bad wheel cylinders, a bad brake line, or is your abs light on?

Albertj

Wheel cylinder? Confused. My riv has disc brakes all around. All calipers are new.

Brake lines should be ruled out due to no fluid leak. I traced them too, they look fine.

ABS light has been on for years and has not operated for years.

Should I look at booster vacuum issues? I hear no hissing .

I noticed that my brake pedal just about goes to the floor under hard braking. I can't get the ABS to kick on if I wanted to. When the car is off, I can get enough pressure built up so that I can't move the brake pedal but when the car is running I can't get much pressure on the pedal. Could this be the master cylinder?

98riv wrote:

I noticed that my brake pedal just about goes to the floor under hard braking.  I can't get the ABS to kick on if I wanted to.  When the car is off, I can get enough pressure built up so that I can't move the brake pedal but when the car is running I can't get much pressure on the pedal.  Could this be the master cylinder?

have you completely bleed the system?

robotennis61 wrote:

98riv wrote:

I noticed that my brake pedal just about goes to the floor under hard braking.  I can't get the ABS to kick on if I wanted to.  When the car is off, I can get enough pressure built up so that I can't move the brake pedal but when the car is running I can't get much pressure on the pedal.  Could this be the master cylinder?

have you completely bleed the system?

Just changed the fluid and bled it.  The pedal also felt spongy before and after the brake fluid change. I just went out and checked it again and I can get some pressure in the lines if I pump the brake quickly but it will then slowly go to the floor.

98riv wrote:

robotennis61 wrote:

98riv wrote:

I noticed that my brake pedal just about goes to the floor under hard braking.  I can't get the ABS to kick on if I wanted to.  When the car is off, I can get enough pressure built up so that I can't move the brake pedal but when the car is running I can't get much pressure on the pedal.  Could this be the master cylinder?

have you completely bleed the system?

Just changed the fluid and bled it.  The pedal also felt spongy before and after the brake fluid change.  I just went out and checked it again and I can get some pressure in the lines if I pump the brake quickly but it will then slowly go to the floor.

Check your brake lines for seepage or outright leaks, replace any damp or rusty lines then replace master cylinder. At your cars age you're probably due.

albertj wrote:

98riv wrote:

robotennis61 wrote:

98riv wrote:

I noticed that my brake pedal just about goes to the floor under hard braking.  I can't get the ABS to kick on if I wanted to.  When the car is off, I can get enough pressure built up so that I can't move the brake pedal but when the car is running I can't get much pressure on the pedal.  Could this be the master cylinder?

have you completely bleed the system?

Just changed the fluid and bled it.  The pedal also felt spongy before and after the brake fluid change.  I just went out and checked it again and I can get some pressure in the lines if I pump the brake quickly but it will then slowly go to the floor.

Check your brake lines for seepage or outright leaks, replace any damp or rusty lines then replace master cylinder.  At your cars age you're probably due.

I replaced my master cylinder and bled the lines again.  As an FYI, the AC Delco unit doesn't come with the bleeder kit so make sure to buy one before you start.  Went for a test drive and it still felt like a little air in the lines.  I saw the below YouTube video and gave it a try and it seemed to help get rid of the remaining spongy feeling.  When should the rubber brake hoses be changed out?  Should they be changed every x number of years or miles?  They look fine on the outside, but don't know how they look internally.

Hi guys sorry to butt in but I'm leaking brake fluid, further investigation the brake fluid is coming out of the brake booster line, that's going from the supercharger to the firewall( leak comes from firewall) next to the brake tub thingy.(sorry still learning) what's going on? Cause that cannot be good at all.

If it is leaking at the back of the master cylinder, it is a seal. Replace the master cylinder.

Ok sweet I was hoping it was something minor

How is the booster rod adjusted on the Riviera?

After replacing the master cylinder and bleeding the system, when the car is very quiet I can hear a slight "tick" when I first press the brake pedal, and I can feel it through the pedal.

I'm relatively certain there is a very small gap between the booster rod and MC, and the "tick" is the rod taking up that tiny bit of slack before pushing on the MC piston.

Some cars have the booster/brake rod adjustment under the dash, others have one screw/unscrew the rod itself with the MC off.

I'm hopeful the rod is adjustable and can be done from inside the car under the dash   

Anyone know if/how the rod is adjusted?

Thanks,
Al

Be sure the 'tick' you hear isn't just the park safety solenoid. I recall hearing a click when I touch my brake pedal. Further investigation revealed that it was the safety switch that releases the shifter so you can take it out of Park.

Good point. The "click" seems to have cleared itself up as of this morning though.

I'll keep an eye, well an ear and foot, out for it if it comes back or not.

Quick question: Any issue with reusing the two M10x1.5 nylon lock nuts that bolt the MC to the vacuum booster?

Mine were still "tight" when reinstalled and I didn't have any new ones on hand to replace them. New MCs don't come with new nuts, so I wonder if it really isn't a big deal?

Al

al_roethlisberger wrote:

Quick question:  Any issue with reusing the two M10x1.5 nylon lock nuts that bolt the MC to the vacuum booster?

No. Reuse them

I just replaced MC. I didn't see how I could bench bleed it since fluid just runs out the tube in the reservoir that connects to the rubber hose (I guess that's for the ABS?). So I bolted the MC in place, then used the bench bleed kit as described using the brake pedal to actuate the cylinder. Worked out ok. I guess if you could plug the bottom of that reservoir tube you could bench bleed the MC but seems easier to do it after bolted on to the car.

Next is replace warped front rotors. Pads are still good, can I use them after rotors are replaced?

If the pads have lots of material and aren't glazed on the surface, I'd reuse them for sure. I've even sold used pads on ebay before.

+1 on reusing the pads as long as they're ok.
Just make sure they don't have a "step" worn in them (from rust), especially on the inboard pad.