Is My Master Cylinder Bad?

[tubbedII]

It was time to bleed the brakes last night, but didn't go well. I'm trying to run a used master I bought off the board, and wondering if it's no good. It was said to be fine and just pulled for a nicer unit.

I put the master on the bench to bench bleed it and filled it about half way. The 1st push of the plunger spit out a little bit of fluid, but after that nothing. It didn't bubble in the reservoir either.

A little frustrated I walked away for about 5 mins and came back to fluid leaking out of the ports. I pushed the plunger and it seemed like it was working but only for about 3/4's of the plunger's push. After that, the same thing, nothing.

Any clues on what may be going on or do I just need to replace it?

[tasian]

it might be bad but it sounds more like you just need to bleed it more you only need to push the plunger in about one inch when your bench bleeding and fill the reservoir all the way full also you said it was leaking out of the ports when you came back? i always have just bench bled new m/c's but they come with a kit that has fittings and tubes to put the fluid back into the m/c without that i would think its going to take A LOT of brake fluid to get all the air out

i'd just keep trying to bleed it the last one i did i spent about 45 minutes trying to get all the air out just remember to push the plunger slowly in about a inch and then let it slowly come back out otherwise if you do it to fast its a bit harder to get all the air out

[tubbedII]

Thanks, tasian. I guess I'll give it another shot...I let it sit over a bucket and will check it out after work so see if all the fluid leaked out.

A little more background is that I didn't start with bench bleeding it at all. My whole brake lines are new, so I just plumbed it all and tried to do without the bench bleed (let my anxiousness take over). We went through MANY (at least 50 or more) pumps of the brake pedal and no fluid left the reservoir. The booster sounded like it was letting air escape through the push rod area on the back as well, so that's why I pulled it apart...

Sorry if this sounds elementary, but do you need to have the vehicle running to bleed a power assist brake system?

[truckdude239]

you need to block of the port before you release the plugger are you will suck air back into the mc

[tubbedII]

Originally Posted by truckdude239

you need to block of the port before you release the plugger are you will suck air back into the mc

That makes sense. I've only blead a mc once and it was over 10 years ago, but I remember little tubes going back into the reservoir from the port that I didn't have last night. I'll search around on how to properly bleed one, I'm sure that's been covered many times. Thanks!

[tubbedII]

3 days of trying to bleed these darn brakes and no luck. I did get the master to bleed where I got all the bubbles out (weren't that many after the gravity bleed), but can't get the fluid to flow through the empty brake lines. I've been reading on the web and I'm going to try the reverse bleed technique. I don't feel like spending a ton of money on a pricey kit, so I was going to try and see if I can get a large syringe from the vet and go that route. Anyone have any experience with this?

[truckdude239]

when i replaced my rear hard line i just opened the screws and walked away for a while i had fuild dripping out the rear screws so i then bleed them like normal after that

[tubbedII]

I read about gravity feeding them like that, but I was a little nervous about walking away too long and letting the reservior run dry putting air in the lines. Given my luck, it would happen...but mainly because I get easily distracted. If I don't find the syringe today, I'm definitely going to give it a shot though...can't really due much more harm than what's happened so far!

Just to share my frustrations with you all in this, I pulled the master off the booster while keeping the lines all hooked up. My EDC is directly under the master, and I thought it was safe to set it there. I walked inside the house and came back to find the master on the ground, lines all bent (no kinks thankfully), and brake fluid everywhere. I cleaned it all up really well, but I'm sure some things will need a re-paint after it soaks in really good. It's all my fault in being overly anxious. I haven't driven the blazer in about 10 months, so I've been trying to move faster. This reminds me to keep taking my time and do it right...

[hatechew]

You can get a hand-pump vacumn set up at the local parts store. It comes with a few pieces of hose and a catch can. Around 20-30 bucks I think. Put it on your bleeder, pump it up, open bleeder, release pressure, close bleeder, repeat. Don' t break seal. It will help pull fluid through the line.

[tubbedII]

Originally Posted by hatechew

You can get a hand-pump vacumn set up at the local parts store. It comes with a few pieces of hose and a catch can. Around 20-30 bucks I think. Put it on your bleeder, pump it up, open bleeder, release pressure, close bleeder, repeat. Don' t break seal. It will help pull fluid through the line.

I saw that at O'Rielly's yesterday too...it was $38. The only reason I didn't get it was because they said if I opened it I couldn't return it and the box didn't have the instruction on the back.

I know you pretty much wrote how to use it, but just to clarify, this pump actually pulls the fluid through the lines from the bleeder? And what do you mean by "don't break the seal"?

[hatechew]

This is a little late. Thought those kits were cheaper than that. Anyways as long as you have fluid in the lines the pump will pull it to the bleeder. Don't break the seal at the bleeder valve till you close it back. This is one of those jobs where you need three hands, because sometimes the hose wants to pop off the bleeder while you are closing it. If you haven't checked be sure your rod is the right length for your master.

[tubbedII]

I borrowed a friends vacuum bleeder that connects to an air compressor to create vacuum, but still no luck...fluid just won't come down into the lines. I went through the lines to make sure there's no kinks or clogs, and everything seems good, so I'm going to try the reverse bleeding trick with a large cyringe now...sure hope it works!

[derail]

I had the same problem with my m/c except mine was new. Drove me crazy trying to figure it out. I did the same thing and followed the lines all the way back to the m/c and couldn't find anything. Reverse bled using syringe and everything. Disconnected the brake line from the master and......nothing. There was a tiny piece of crud in the hole in the bottom of the m/c. I couldn't see it. I used a small piece of wire and fished around the in the hole. Pretty soon fluid started flowing and it was fixed. Just a thought.

[tubbedII]

Originally Posted by derail

I had the same problem with my m/c except mine was new. Drove me crazy trying to figure it out. I did the same thing and followed the lines all the way back to the m/c and couldn't find anything. Reverse bled using syringe and everything. Disconnected the brake line from the master and......nothing. There was a tiny piece of crud in the hole in the bottom of the m/c. I couldn't see it. I used a small piece of wire and fished around the in the hole. Pretty soon fluid started flowing and it was fixed. Just a thought.

Did you bench bleed the master? I'm just assuming and hoping that since I'm able to get good flow when bench bleeding, there's no issues with the mc...

[derail]

Originally Posted by tubbedII

Did you bench bleed the master? I'm just assuming and hoping that since I'm able to get good flow when bench bleeding, there's no issues with the mc...

I did bench bleed it before I installed it. I did it again to be sure after I figured out that fluid was having a hard time getting to the rears. Just thought it might be something to think about if you don't think fluid is getting to the bleeders. I also used a 60cc catheter tip syringe with clear plastic tubing pushed onto the tip and reverse bled them that way.(My sister works at a pharmacy)

[LostMy65]

I'm going to plug the ports on the master cylinder to see if it holds a strong pedal to eliminate the question if the master cylinder is bad or not.
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[tubbedII]

Ok...with my direction lately been devoted to sanding and prepping for paint, the brakes kinda fell down the list of priority. I did get another master and prop valve (still used but it was definitely off of a working vehicle), and I just ordered a set of speed bleeders for it. I'm hoping this does it! Once the bleeders come in I'll put it all on and let ya guys know how it went...

[LostMy65]

I plugged off both ports on the master and I have a rock solid pedal. So that tells me my master is fine.
I then hooked the rear line to the rear brakes master port but left the front plugged, and I still have a good pedal.
I then hooked up the front line and the brake pedal went soft when I started the engine. So that tells me my issue is with the front. All these tests were with the engine running.
I'll do some checking with the front and see if I can locate the issue and then I'll post what I find.
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[LostMy65]

Done for the day.
I didn't resolve my issue.
I feel better knowing the booster and mc are fine.
By plugging off the port to the front brakes I get a firm pedal, so the issue is with something in the front.
Next I'll try bypassing the proportioning valve and see what happens.

[tubbedII]

Glad you're getting yours dialed down! I got my speed bleeders in, but unfortunately my rear were 2 different sizes, so I still haven't tried it. Gonna order the right size today and hopefully have a firm pedal by the weekend!