master cylinder question

My brakes are feeling a little spongy. I was doing some research & came upon "the master cylinder is leaking internally". I imagine this means there is a leak I cannot see. So how do I check this as a possibility? & curious about this in general. Where does it leak to?

 

Fluid seeps by the check valve or plunger cups/seals and returns to the reservoir. You wont see a fluid leak. Air in the brake system causes a spongy pedal also. Brake fluid absorbs moisture and turns dark in the reservoir. A brake fluid flush is a good place to start if your fluid isn't clear.

 

Gotcha.... Thanks for explaining that....
I did a complete front brake job this past spring, so I hope it's still clean, although I prefer a flush to a pump change.
I'll check the fluid color tomorrow....

 

Does your brake pedal slowly go to the floor under light pressure or is it just spongy and doesn't go to the floor?
@Kristi with a K

 

Door #2 - just spongy and doesn't go to the floor

 

get a long zip tie (about 7 inches) and stick it down into the brake booster, if you pull it out and there is brake fluid on it, the master cylinder is leaking!

 

Flushing the brake system by bleeding should do the trick. You can do it by yourself by attaching 22" of 3/8" clear vinyl hose to the bleeder valve. Put the other end in a quart jar with an inch of brake fluid in jar to cover hose end. Open the bleeder valve and hand pump the brake pedal, engine off. After several pumps fill the reservoir and repeat. Leave the hose on bleeder valve and in jar and close bleeder valve. Move the hose/jar setup to next wheel. The Brake bleeding order rr, lr, rf, lf.

 

I've only had to bleed the fronts, although I do understand that when I perchance to do the rear brakes that I will have to also bleed the front. I notice you did not include the BPV. Not necessary?

 

Do you have a trick to keeping the tubing on the valve? It's a PITB once it gets fluid on it....

 

The BPV is usually way to rusty to risk damaging and I've never had to so I don't know from experience and have heard bleed it last and bleed it after lr.

 

K. Had to do my homework & look up the brake booster location. Thanks for the tip & I'll check that tomorrow as well....

 

I heat the tubing end with a lighter and push it past the flare on bleeder valve. Once it's shaped to fit it stays on for me and pops onto the next easy. Be sure to replace the bleeder cover caps when done bleeding.

 

Rubber vacuum hose would probably stay on the bleeder valve better than clear vinyl. Silicone vacuum hose better still. I've gotten used to clear hose as you can see any air bubbles while 2 person bleeding.
@Kristi with a K

 

If you have ABS it's entirely possible you have trapped air in the ABS block, only way to bleed it out is to take it into a shop that has techstream and can open and close the ports in the block through a diagnostics tool.

If your brake pedal never goes to the floor then most likely your master cylinder isn't leaking.

 

In our part of the world trying to bleed the LSPV can turn into a very expensive and time consuming project.

It gives one a real good reason to remove the LSPV from the system and if wanted replace with a manual valve .

 

Is there a write up on this? I have a friend who wants to look into it as a possibility.

 

I am sure with some searching you can find some.

I have seen threads about them.

It is not all that hard people seem to think they need to go in the back under the truck for some strange reason.

The same ones that love to crawl under the truck to check the air in the spare

I put mine right at the Master Cylinder run one new line to the rear . plug the line where it Tees at the front.

I have a valve to put on just not that high on the list about 60,000 miles now

 

OP,
Your best bet regarding the LSPV... ditch it and go with an aftermarket unit. All of the plumbing is back there to mod and make it work. Install the new unit near the MC. Its out of the weather, adjustable and allows you to remove all of the OEM garbage in the rear that is visible and looks awful.
Here is mine for example:

 

Minus my shock re-location and ARB line, once you've painted everything up nicely, you should have the extended brake line and parking brake cables headed to the axle.
Just looks like a cleaner set-up to me and less crap to get hung up on things.