Hot Rods Another master cylinder question

So what you are telling me is that the MASTER CYLINDER is sealed and there is no reason to add fluid to the drum brake section (DISC/DRUM MC) to compensate for lost fluid volume of the MC? What about a SGL RSVR MC. Is it hermetically sealed also?

So, when your car is in for service and on the lube rack and the kid is checking all fluid levels, it would be wise to advise him to forgo the brake fluid as it 'already factored in' and there is no need to check it?

 

The extra fluid to move the shoes against the drum, when the shoes are not adjusted tight, and there is a lot of pedal travel, is the fluid that is in the master cylinder bore. You just push more of it out into the brake lines, than you would if the shoes were adjusted properly. This fluid goes back into the master cylinder bore when you release the pedal.

It has nothing at all to do with the reservoir.

But, to answer your question about why you need to check the brake fluid and keep the reservoir full, is because any fluid that leaks out of the system anywhere, will make the level in the reservoir go down a bit. This is because when the pedal is released, there is a small opening that connects the reservoir to the master cylinder bore.

 

And the PRESSURE RESIDUAL VALVE(s) has no bearing on returning fluid required volume? Not all fluid returns to the MC piston(s) bore.

How the brakes are adjusted has no bearing on fluid volume. Simply put, as the brake linings wear, it is going to require a larger fluid volume to push the pistons.

Without proper volume, you are going to have a soft pedal (same for rear shoes out of proper adjustment) at some point. If no fluid is ever added, at some point there will not be enough fluid volume/pressure to fully apply needed shoe pressure against the drum(s).

 

Key sentence here. So what you are saying here is that when fluid volume is insufficient in the MC piston bore, the compensating port in the reservoir adds additional fluid to the piston bore?

Now where does the needed fluid replacement in the reservoir come from if there is no need to top-off the reservoir regularly?

 

You only need to top off the reservoir on a drum brake system, if it leaks out somewhere.

But you'll see the level in a disk brake system drop over time as the pads wear, and they have a large reservoir so you don't need to top it up.

 

So what you are saying in essence, is that brake shoe linings never wear?

 

You should never work on brakes with your understanding of how they work!

 

Wheel cylinders return to the same retracted position when released, so unless there is a leak somewhere, the fluid level remains the same as opposed to disc brake calipers that stay extended and retain fluid as the pads wear lowering the level in reservoir.

 

And you should never work on them due to your misunderstanding of how they work.

 

OK, lets try this again.

I understand the concept you guys (some) are describing but in reality it doesn't work that way.

Within the WC, you have two opposed pistons. Hydraulic fluid pressure/movement initiated by the MC moves the pistons outwards and forces the shoes (at some point) against the brake drums. That same amount is returned to the MC on brake pedal release if the MC internal return spring, the WC components and shoe return springs are within spec.

Now the problem begins with as the brake linings wear, the pistons have to move a greater amount of travel and additional fluid volume (from the MC reservoir) is needed to fully apply the shoes against the drums. At some point, that extra needed fluid has to be replaced as the increased needed fluid volume is needed to properly move the WC pistons further outwards. This is the purpose of the compensating valve in the MC. It will allow more fluid to be introduced as needed into the MC piston bore and brake lines to fully extend the pistons. Simply put, a used brake system will need/have a greater fluid volume than one is put into service initially. When the system is serviced, it returns to the original assembled volume (if O/S PISTON WC's are not used and the MC piston bore has not been sleeved.)

That returning fluid will not refill the MC piston bore/reservoir (well, it will for that one brake application as the lining wear continues, a greater volume is continually needed). That is where make-up fluid in needed (considering no hydraulic leakage and you put the MC cap back on). The system fluid capacity is increased due to continuous lining wear.

That will create a greater volume needed in the system with the brakes applied. That is the additional purpose of the MC reservoir to allow make-up fluid due to greater WC piston travel due to brake shoe wear. It is the same principle found on disc brakes.

The only way for the brake fluid volume to remain the same is to not drive the car to experience lining wear.

You see all these misunderstandings all over the MESSAGE BOARDS. It is due to those that keep repeating old wives' tales to the newcomers. It seems the lack of common sense is more prevalent here as many work on older application or are trying to bring them up to current tech but have no idea of how to go about it and getting incorrect advice.

I have even seen trained experienced mechanics have no idea of how a MC or valving works. They have no idea of how to diagnose a system. They point a parts cannon at the car.

I have been working on cars since 1966 (I know - whoopty-do). I have not lost one tooth, stretched a seat cover, had to replace any sheet metal due to a poorly serviced brake system or was sued.

OH! BTW ... When you have the car up on a lube rack. do not forget to grease the bleeder screws. I have seen that one also.

What you think of me personally I have no concern. In fact, send me your PayPal ACCOUNT to send you a quarter to let you call someone that cares.

 

As the fluid level drops as the front pads wear, fluid in the reservoir drops as a result of pad wear. So at what point will the reservoir that is never filled fail to be able to move the caliper pistons?

 

In a non-leaking system, the disk brake reservoir will have enough fluid to push the pads out till the linings are worn out with a small amount still remaining, NO fluid will need to be added! When new pads are installed, you can compress the caliper piston back, refilling the reservoir with the original fluid, or open the bleeder and push the old fluid out, then refill the reservoir with fresh fluid.

Drum brakes are self adjusting, so as the linings wear, the self adjuster turns spreading the shoes outward. The wheel cylinders always move with in a short range never going past the point of the self adjuster clicking over. If you could watch a time lapse of the fluid level as drum brakes were, you would see the fluid level fall and rise as the shoe get worn and adjusted, but it will always stay in an almost full position never needing to be topped off.

Both statements are assuming all components are engineered too work with each other. Mismatched parts do not count!

 

And it just keeps going ...

Sorta like that crazy rabbit in sunglasses and flip-flops beating on a drum.

 

"So at what point will the reservoir that is never filled fail to be able to move the caliper pistons?"
After everything is worn way beyond anything the engineers that designed it intended it to be.

There is a lot of good information in this thread. There is also a lot of misunderstanding happening here too. This thread would be funny if it weren't such an important subject.

 

With drum brakes every time you push the pedal all of the fluid is returned to the master cylinder by the springs on the brake shoes. As the shoes wear the pedal just goes down farther, until the brakes are adjusted. As always all the fluid used to expand the shoes is returned to the master cylinder. With disc brakes there is no adjustment as the pads wear. The calipers just push the pistons out farther but they do not retract like drum brake wheel cylinders do. So the fluid in the master cylinder for disk brakes goes down as the pads wear. The master cylinders for each system are designed with a fluid reservoir holding enough fluid to compensate for pad or shoe wear.

 

And there is mostly bad. And this is not the only thread here that gives bad information. It is just like a plague.

I see no humor as these cars are most likely being operated somewhere close to you.

 

So in your world, the caliper pistons do not retract. Kinda expensive for continual pad/rotor replacement isn't it?

 

The pads do not retract, they are knocked back a few thousands by the rotor runout. Nothing in the brake system retracts the pads, no springs nothing. You need to go to another forum where people will believe your mis information. I'm done here. Have fun.

 

BJR, post:

"The pads do not retract, they are knocked back a few thousands by the rotor runout. Nothing in the brake system retracts the pads, no springs nothing."

Uh, the piston(s) is retracted into the caliper bore by the piston seal.

If you have rotor run-out you will have brake pedal vibration(s) on brake application and with no piston retraction dragging pads, as well as a soft pedal. The pads are attached to and move with the caliper (depending on caliper design).

"Nothing in the brake system retracts the pads, no springs nothing."

You need to go to another forum where people will believe your mis information. I'm done here. Have fun."

THANK GOD YOUR DONE.

You need to pick up a BRAKE SYSTEM TECH MANUAL and try to read and comprehend if possible.

HINT - Try reading without moving your lips and you can use your finger to identify syllables.

This is the lunacy that keeps being posted here.

 

Original poster had a ligetmate question and Jim and BJR had good responses. But, as many of the posts here, it became a pissing contest which eventually overshadows the real issue.

 

A drum brake MS? turns in a disc brake MS discussion.
Par for the course

 

I was taught that the rear brakes need to be activated first. This is to set the attitude of the vehicle and apply downward force to the front wheels.

 

You were taught wrong.

 

Did we figure the original port question out yet?

 

The more I think I've learned about my original question, the less I think I know.

 

My ignorant question. If there is no difference on dual reservoir DRUM master cyls (watch someone bring up disc MSs again now) why did the factory designate them?
Something further down the line?
Is the “timing” mentioned real?
Crap I don’t know.

 

There is no standard, it depends on the manufacturer, no the timing is not real.

 

And it keeps on giving ...

Go here - https://www.brakes-shop.com/brakepedia/general/brake-bias-and-performance

And scroll down to -

The ever-present weight transfer phenomenon

What part of 7th grade physical science class did you misunderstand?

 

And I should never work on brakes?