Firebird Clutch Master Cylinder

For the past month or so the clutch pedal in the Firebird has been acting weird.  Sometimes it would be normal but other times you’d feel the resistance stop before the pedal was all the way up.  Then, when you went to press it down again there’d be a greater zone of no resistance (aka play).  It’d come and go randomly.  The clutch system in the Firebird is hydraulic and consists of a master and slave cylinder.  The master cylinder has a rod that is depressed by the clutch pedal.  It is located in the firewall near the clutch pedal.  The slave cylinder is bolted onto the front of the transmission around the input shaft.  A hydraulic hose connects the two.  The stock clutch master cylinder has design issues with it that you can read about online and isn’t regraded fondly.  The slave cylinder has a good reputation and the stock one is used in high power applications without issue.  There is also a reservoir that holds extra fluid and connects to the clutch master cylinder.  It isn’t under pressure.
I’ve had to replace the clutch master cylinder before.  It was leaking and would slowly drop the pressure in the line resulting in the clutch slowly reengaging even when you had the pedal all the way pressed down.  You can test for a leak by finding some place level, putting the car in first gear, foot off the brake, and holding the clutch in.  If after a little bit, you start moving forward , you have a leak.  So, go look around your clutch master cylinder.  That time, I could see fluid leaking around the rod when I stuck my head under the dash.

Unfortunately, the clutch master cylinder can also leak internally.   By this I mean that inside of the cylinder, fluid can seep past the piston and return to the line going to the fluid reservoir.  When this occurs you’ll lose pressure but you won’t see any leaks.  The clutch slave cylinder can also leak.  If it does, you may notice fluid leaking from the bits of it that protrude out of the bell housing or fluid leaking from the bell housing.  My car leaks oil which is all over the bell housing and transmission so I wouldn’t be able to tell if it were leaking.

Using the time tested method of narrowing the problem down to two things and picking the simpler thing, I decided to replace the clutch master cylinder.  I wasn’t able to take a ton of pictures because of where it is located and my hands were messy.  So, this will mostly be verbal.  Out of the ordinary I know.

Anyways, there are a couple of panels that need to be removed to get under the dash.  After you do that, it looks like this.  The thing being pointed out with the arrow is the clutch master cylinder.  The rod that extends out of it has a circle on the end that attaches to the clutch pedal arm near the pivot point.  It is retained by an E clip.  Remove the E clip without shooting it up into the dash and remove the end of the rod off the stud.  Next, remove the two nuts that are beside where the clutch master cylinder comes through the firewall.


The rest of the clutch master cylinder can be found under the brake master cylinder.  It’s the black cylinder pointed at in the picture below.  Note this picture is taken from in front of the car which isn’t where you’ll be if you try to remove it.  Thanks to GM, you get to do this completely sight unseen.  Now you need to remove the hose from the clutch fluid reservoir.  It is held on with a zip tie which you can just snip to remove.  With a rag at the ready, pull the hose off and mop up the leaking fluid.  The clutch fluid is actually brake fluid.  Brake fluid and paint don’t agree with each other so keep it off your paint.   Go ahead and remove the reservoir as well.  You’ll need the room.  Removing the clutch master cylinder is the easy part though since you just pull it out from the firewall.  The two nuts that you removed earlier under the dash don’t go onto bolts.  They go on to a U shaped piece of metal that has threads and flanges at the end.  You’ll need to pull it out of the clutch master cylinder before you can proceed under the car. 


Once the car is jacked up and on stands. look near the driver’s side exhaust manifold and follow the hydraulic line up.  First you’ll need to move the metal heat shield on the fuel and brake lines out of the way.  Take the nuts that hold it in place off and bend it out of the way without creasing it.  Now reach up and move, jiggle, rotate the clutch master cylinder to get it out from between the steering linkage, body of the car, and the fuel/brake lines.   Now, only the hydraulic hose keeps it connected to the car.  The hose is held in by a small roll pin near where the line connects.  Drive it out using a punch and hammer.  A piece of wood with a small hole in it works well to hold the clutch master cylinder while you drive the pin out.


Yay, it is finally out.  Here’s what it looks like.  There are two holes on the flange that the U shaped bolt goes through to hold it on the fire wall.  The rubber hose goes to the reservoir and the elbow looking part on the right end is where the line to the clutch slave cylinder connects. 


I returned from the parts store a little surprised.  The replacement part was made of metal while the previous one (and the other one or two I’ve replaced before) have all been plastic.  It was the right part number though.  The metal one was a little bigger than the plastic one which had me worried because it was tight getting the old one out.


Re installation is the reverse of removal but it goes slower and involves more choice words.  Under the car, replace the rubber washer on the end of the hydraulic line and reinstall it with the roll pin.  Now get the clutch master cylinder sitting beside the steering linkage then go inside the car to pull the rod through the hole in the firewall.  Back under the car for more struggling until it is on the other side of the steering linkage in approximately the right place.  Now the fun starts.

Inside the car is a bracket that limits how far the clutch pedal can move down.  If you look back at the first picture and the silvery piece near the clutch master cylinder, you’ll see it is also held in place by the U shaped bolt.  This piece won’t sit in place to line the holes up so you’ll need to find a way to hold it in the correct position.  If you don’t, it will block the holes in the firewall and keep you from pressing the U shaped bolt in.

Now, go to the driver’s side of the engine bay.  You’ll need to reach your right hand around and under the brake master cylinder while your left hand goes under the flexible fuel lines and above the steering linkage.  Now you can inefficiently move the clutch master cylinder around but you’re not able to push it that hard.  Now move it into the right position while trying to match the holes in the clutch master cylinder and fire wall up…which you can’t see.  Once you’ve got that, hold it with your left hand and try to put the U-shaped bolt in.  If you try to put the U shaped bolt in before getting the clutch master cylinder in place it will hit the brake master cylinder.  Previously, I’ve shoved and pushed and finally gotten the U shaped bolt in enough to put the nuts on it and draw it into the correct position.  That wasn’t happening this time.

Try as I might, I could not get both bolt holes to line up at the same time no matter what I tried.  I finally decided that the cylinder of the clutch master cylinder was a little larger in diameter than the old one.  As such, it was hitting the top of the hole in the firewall keeping the last bolt hole from lining up.  As opposed to grinding the opening a little larger I filed the bolt hole a little larger.  After that I was finally able to get the U-shaped bolt back in place.  Now put the nuts back on and re assemble the dash.  Also reattach the reservoir hose to the reservoir with the supplied zip tie.  Swapping the part is done.  Now on to bleeding the system.
Since you opened up the hydraulic system there is now air in the line.    Top off the now empty reservoir with fresh fluid.  You’ll see some bubble and the fluid level will drop.  Add a little more until that stops.  Now to get the rest of the air out.  There are multiple ways of removing the air.

The “regular” method  requires two people.  One of them pumps the clutch pedal vigorously numerous times and holds it down.  The second person is under the car and opens the bleeder valve connected to the slave cylinder.  See it in the middle of the picture below.  Opening the valve will shoot fluid onto the underside of your car.  So, put a hose on the end of it that leads down to a cup of some type.   Once it is done dripping snug the bleeder valve closed.  Your assistant can then let up on the pedal and start pumping again.  Repeat as needed until the pedal feels like normal.  This method may or may not work on this system.  Apparently, something in the design makes it trap air.

The second option is called vacuum bleeding.  You apply a vacuum to the clutch fluid reservoir while you assistant vigorously pumps the clutch pedal.  After a bit it will stop coming up all the way and you’ll have to pull it back up.  Keep pumping.  Let the vacuum off and pump the pedal until pressure builds back up.  Repeat as needed until you get a good pedal.   For vacuum you can use a hand pump and fab up a way to create an air tight seal on the reservoir.

The third method is called gravity bleeding.  It has the advantage of needing only one person. I can be slow though.  It usually works well.  All you do is take the top off the reservoir and crack the bleeder valve open until you get a decent rate of drops coming out.  Leave it this way and periodically monitor the fluid level in the reservoir.  Keep it topped off because, if it gets low, air will be sucked in and you’ll be back at square one.  Keep this up for a while and periodically check the pedal.
You can use all the methods.  I usually use the first to get a large amount of air out of the system and to remove the old fluid.  Then, I use the second and/or third method to finish it up.  Once you’re convinced you’ve got a good pedal for for a test drive.  If everything is good you’re lucky.  If the pedal doesn’t stay the same you probably still have a little bit of air in the system that has moved from wherever it was hiding to cause trouble.  Go bleed it again.  You may come back the next morning and find that the pedal is not good again.  I don’t know why, but lots of folks say it happens.  Bleed it some more.  Once you reach the point where the pedal is good enough to operate but not perfect drive the car a bit.  The motion of driving the car will shake the last little bit of air out of the system.

I’m currently at this point.  The clutch feels good and operates like normal but isn’t quite right.  When I let off with my foot the clutch pedal will stay still and then slow move the last little bit into place.  I’m not sure if there is a tiny bit of air left in the system or it is related to the metal clutch master cylinder.  I’m going to drive it in hopes that if it is air, it will work its way out.
I’ve also recently noticed the air conditioning compressor rattling on the car.  I don’t think this is a good sign.

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