So I installed a DRM brake bias spring in my 84. I was hoping to get lucky and not need to bleed the master cylinder by itself. I figured it would be a long shot. But I used a vacuum pump to pull the fluid to the passenger side rear caliper to start. I started to get just a tiny bit of fluid but it didn't look right. It was only a dribble. I had my son pump the brakes for me thinking that my pump was bad. When he would push the peddle down only a drop or two would come out. I tried the drivers side and it was the same thing. So I tried the front passenger side. It seemed to bleed like normal. I then figured I would need to bench bleed the cylinder. I hooked up my cylinder bleed fittings and clear tubing. I noticed right away that the rear port was hardly moving any fluid. So I disassemble the cylinder. I remove the spring and check for any problems. Everything looked fine. I put everything back together and try again. Now the opposite is happening. The rear port is moving all kinds of fluid but the front two ports are hardly doing anything. They can't clear the air out of the lines. I'm about to put the factory spring back in and see what happens. What am I missing? What would cause the spool to stick like this? This master cylinder is only a year old and is still really clean inside. It also worked fine before the DRM spring.

 

Well just for grins I swapped the factory spring back in. No change. I still have almost no movement of fluid for the front brakes. The odd thing is if I step on the peddle fast enough it shoots fluid up and out of the bottom of the reservoir. Only on the front though. The rear works like it should.

So either I have this thing assembled wrong or I broke something. I'm at the point of throwing in the towel and buying a new master cylinder and giving up on the DRM spring.

 

The fluid shooting up in the reservoir is normal/typical. This happens as the master cylinder piston's lip seal is moving past the compensation port before fully closing off the port. The rear may have a center valve instead of a compensation port. Not sure about the actual master cylinder design details, however.

 

So dumb question. Whats the purpose of the screw on the side of the master cylinder? Is this a bleed screw? To remove the spool from the MC so I could install the DRM spring, I removed that screw and use compressed air to push out the spool. I just assumed that's what the screw was for. But can you bleed the MC with that somehow?

 

Do you mean the port that is for the differential pressure switch (for red brake warning lamp activation)?

The position of the valve is important and may cut off flow to the low pressure circuit as an additional safety feature.

 

No I mean this.


This is what it looks like removed. I pulled this one from my old MC.



To remove the spool I removed this pin after I drained both cups of fluid. With the spool cap off I blew compressed air through the port that this pin was blocking. This pushed the spool out without trying to pry it out. Anyway I'm fairly certain this is a bleeder. With the MC in a vice on my work bench and the bleeder tubes on it I cracked that bleeder open and pressed the plunger in. Air and fluid came out the bleeder. I tightened it before allowing the plunger to return. When I did that I saw both front and rear tubes moving air. So I did that a few more times. Now both front and rear ports are moving fluid. Being that the 84 MC is a one off I wonder if this is a specific issue with them? I never knew that you would need to bleed the master cylinder with a bleeder screw. I didn't do that when I first installed it. I just bench bled like normal. But as of now the MC is bench bled. If I get some time tonight I'll hook the vacuum pump to the calipers and see if I can get his thing all done.

 

Looks like that screw is just a stop for the primary secondary piston (sorry about the small image size):

 

That makes sense based on the shape of the pin. But either way as soon as I loosened that screw and pressed the plunger in I then had flow front and back. So even if its a stop for a piston it worked like a bleeder. Also removing that pin and using compressed air to remove the spool is way easier than trying to fish that thing out with a pick and needle nose pliers.

 

Note the disassembly instruction #3 and installation instruction #8 in the 86 center valve recall notice:

https://www.corvetteactioncenter.com...valve-895.html

 

No issues bleeding with DRM spring (on a 1993)

 

No problem on my 94. Make sure the proportional valve is centered.

 

I bench bled mine first. I put the spring in, then on a vice pushed it in to get air out.

 

Thank you @tequilaboy for all the diagrams. They helped a lot.

So I finally got the rear brakes bled. I had two things working against me.

When I installed the DRM spring I couldn't get the spool out of the MC at first. I was afraid that I would scratch the inside pretty badly by trying or I would trash the threads for the end cap. So I pulled the pin stop out (I thought it was a bleeder) and I blew compressed air through the hole to push the spool out. It pushed the spool out without damaging anything. I think the problem came when I put that stop pin back in. I don't think the secondary piston was in the right position. I think somehow I limited its travel with that stop pin. I think this because when I was trying to bench bleed it the plunger wouldn't move very far. So when I loosened the pin screw and then pushed the plunger in it started moving fluid. Then I tightened the pin screw while holding the plunger in. Then I bench bled it like normal.


With the MC back in the car I ran into problem #2. Not only did I install the spring, but also a reverse line lock solenoid at the same time. I've had it mounted in the car for a while but I never had the lines run to it. For those that don't know a reverse line lock is a system that stops the rear brakes from getting pressure as long as the button is pressed. This is far superior to a "launch control" line lock IF your purpose for the line lock is to help you do burn outs. I tried using a vacuum pump to bleed the brakes through the B&M solenoid. I couldn't get much fluid through it. So I thought the problem was with the MC. Turns out the vacuum pump I was using just didn't have enough *** to pull fluid through the new lines and that solenoid. So tonight I convinced my wife to help me bleed them the old fashion way. So now the rear is bled and I tested the line lock. It all works as it should.

So now that the weather is getting nicer I'm hoping to kill off the tires I have on it with a little help from the line lock so I can get my new set.

Thanks guys for the help.