Sticking Piston in Brakes Master Cylinder

SnowStorm34 · Jul 2, 2016

Hello everyone! This is a continuation from this thread: http://forums.yesterdaystractors.com/viewtopic.php?p=8020109#8020109

Long story short, my home built tractor's brakes don't work. I figured I needed to start by rebuilding the master cylinder. I sourced the parts, rebuilt the MC, and bled the brakes with the help of my neighbor. (He has a tool that sucks fluid out of the brake lines from the bleeder valve.)

Anyway, the piston inside the master cylinder keeps sticking. I cleaned and honed the cylinder, lubed the pieces from the rebuild kit before inserting them, and was careful to make sure everything went in correctly.

Any ideas why it might be sticking?

I appreciate any feedback I can get

old · Jul 2, 2016

Sticking makes me thing wrong piston or cylinder bore not a true round or the piston is miss shaped

Steve@Advance · Jul 2, 2016

A few things come to mind.

The rebuild kit, did you compare the length of the new spring to the old one? I've gotten them that were too short and would not return the piston.

When you cleaned, honed, and lubed the parts, what did you use? When cleaning, and honing, don't want to use any petroleum product! Any thing petroleum based, naphtha, diesel, kerosene, gasoline, etc. will swell the rubber seals. Even the residue will swell them. Only use non petroleum, like brake cleaner, lacquer thinner, acetone, etc. For lube, same applies, there are special rubber lubes, I just use brake fluid.

How is the pedal returned? There needs to be some means to return it. If the weight of the pedal is resting on the piston it may not want to return.

Bob · Jul 2, 2016

I'm gonna GUESS the rubber piston cup is not sliding back well in the honed bore because it is too rough.

So, the piston, on the other side of the spring from the rubber cup, isn't returning, either.

I would try to identify the master cylinder and replace it with a new one.

SnowStorm34 · Jul 2, 2016

Thanks for the replies, everyone.

As someone who isn't very mechanically inclined, I really appreciate all the advice and info.

To further update the situation, I decided to pull back the boot and take a look down the cylinder. To my surprise, the piston was right up against the snap ring. There is a lot of travel in the pedal before the pushrod makes contact with the end of the piston, so I figured the piston was stuck somewhere down the cylinder. It appears that's not the case.

Could it be that both wheel cylinders are stuck closed? (I don't know if it would be considered open or closed. Regardless the wheels roll as if the brakes are not engaged.)

The other thing is where the main brake line splits in two.

A long time ago, my Dad and I adjusted the bolt thinking we were bleeding the system. Does this seem like anything? I wonder if this bolt has the ability to close the line, and maybe it's open only a hair...

Again, I'm really thankful for everyone taking the time to help me with this. If anyone has any further input, I'd really appreciate it.

SnowStorm34 · Jul 2, 2016

One more thing...

Sorry to double post, but to answer Steve@Advance's questions, the spring in the rebuild kit was about 1/8" longer. I figured out that this particular single pot MC was very common on early 60's Ford cars equipped with manual, drum brakes. Sourcing the correct rebuild kit was easy.

Also, I just used brake fluid to clean and lube all the parts. I actually found a PDF of a shop manual for the 1964 Galaxie which outlined how to rebuild the cylinder. It said to only use denaturalized alcohol or brake fluid for cleaning.

Steve@Advance · Jul 2, 2016

So, do you now have a good, hard pedal? And no braking action on the wheels?

As I recall,the bleeder ports were inaccessible with the fenders on... Did you get to them and bleed the system?

If you did, and got pressure at the bleeders, then you are getting fluid to the cylinders.

If you are getting a good pedal, pressure to the cylinders, and still no braking, then either the cylinder pistons are frozen in the bore (rebuild or replace the cylinders), or the shoes are soaked with fluid (rebuild or replace the cylinders and replace the shoes).

The bolt on the lines is just there to hold the fitting in place, no effect on the flow. Now there could be a restriction in the rubber line, not likely but possible. Again, that goes back to what happens when you open a bleeder port. If you get good fluid flow, you don't have a restriction.

SnowStorm34 · Jul 4, 2016

(quoted from post at 21:46:01 07/02/16) So, do you now have a good, hard pedal? And no braking action on the wheels?

As I recall,the bleeder ports were inaccessible with the fenders on... Did you get to them and bleed the system?

If you did, and got pressure at the bleeders, then you are getting fluid to the cylinders.

If you are getting a good pedal, pressure to the cylinders, and still no braking, then either the cylinder pistons are frozen in the bore (rebuild or replace the cylinders), or the shoes are soaked with fluid (rebuild or replace the cylinders and replace the shoes).

The bolt on the lines is just there to hold the fitting in place, no effect on the flow. Now there could be a restriction in the rubber line, not likely but possible. Again, that goes back to what happens when you open a bleeder port. If you get good fluid flow, you don't have a restriction.

Thanks so much for your response, Steve.

The pedal is solid. Literally. :lol: I can only push it down 1/4". That's why I thought originally the master cylinder's piston was stuck, but I don't think that's the case anymore.

The fenders are off and the system has been bled. Using my neighbor's tool that hooked onto the bleeder valve, we sucked out the fluid until it looked clear and fresh. Though I'm starting to wonder if the pedal was pushed down far enough when we were bleeding the system. The amount of pedal travel before the push rod engages the piston may have thrown us off. Could a hard pedal be a symptom of that?

Either way, tomorrow night I'll try opening the bleeder port. That should answer the question of a blockage as well as if the system was improperly bled the first time.

Sticking Piston in Brakes Master Cylinder

SnowStorm34

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old

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Steve@Advance

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Bob

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SnowStorm34

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Steve@Advance

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