VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 Drum brake woes...how to check master cylinder?

My lack of knowledge is going to show here. I backed my 1940 Master 85 out today for it's weekly 15 minute run. I notice that the brake pedal just goes straight to the floor without any resistance! I had someone sit in it with the brake down and I could push the car around.

I see the arm moving when you press the brake into some rubber circle and I assume it's attached to the master cylinder. However, that metal box is so deep in there I don't understand how I would even check it to see if it's empty.

How do you check and fill the master cylinder?

Lift the floor mat under the drivers feet. There's a large rubber plug you remove to access the top of the master cylinder.

http://chevy.oldcarmanualproject.com/parts/1929_41/41cp106.html

It might look like this.

Wow! I pulled the rubber plug up and I see the top of the cylinder. Unfortunately I dont have a socket that gigantic. Does anyone know exactly what size it is and I'll pick one up that size tomorrow. There was 4 flat head screws on the top should I take those out instead? Seems like the plug is the better option.

I know nothing. However, I would leave the screws alone. You don't want the top off of that M cyl while it is in the car. You can worry the cap off with a crescent wrench or a pair of channel lock pliers. Yes, it is a very inconvenient location for an M cyl.

As a suggestion, clean the floorboard and the top of the M cyl. well. You do not want anything falling into the cy. That will cause problems you don't want later.

As you know, if the M cyl is low on fluid, you do have a leak somewhere in the fluid system. Fill the M cyl and hope you didn't let the fluid get low enough in the M cyl to get air out into the system. Otherwise, you'll have to bleed the air out. If you must bleed the system, do NOT push the brake pedal over 1/3 of the way to the floor. Otherwise, you will push the piston seals into an area in the cyl bore that will ruin the M cyl piston seals and wind up having to rebuild or replace the M cyl. Ask me how I learned that!

I've done several brake jobs over many years. However, there are those on here that can advise you much better than I.

Good luck -

Ugh. I assume since the brakes are not working at all that if the problem is the fluid level that it must be all the way empty. Is there some scenario where air isnt in the brake lines but the fluid is so low that the brakes no longer work?

I also assume to bleed the system the wheels would have to come off to get to where the bleed valve is? I've changed and bled many disc brakes but never touched a drum...

With your pedal on the floor and no brakes, I would assume you now have air in the system. It might be possible there is enough fluid in the M cyl bore to have prevented air getting into the system. If, when you fill the M cyl, you again have brakes, I would consider you very lucky. (Remember, DO NOT push the pedal to the floor after you fill the M cyl.)

If it were me (and it ain't), I would fill the M cyl and gently push the pedal a few very short strokes to see what happens(put the cap back on the M cyl first). In any event, you should again have some sort of braking ability - even if it is spoungy and uneven at the wheels (air in the system).

'Been many years since I even looked an one of these M cyl's. I don't recall if they have a bleeder screw or not. For starters, you might hope the air is only in the M cyl bore and not out into the system (which is doubtful). With that thought, you could get someone to put gentle pressure on the pedal and see if you can get air out of the M cyl. Even without a bleeder screw, you could loosen the stoplight switch or the outlet line enough to allow fluid/air to escape (don't take either one all the way off). I think this is a long shot but might be worth a try.

As to bleeding the wheel cylinders, the bleeder screws are on the outside of the backing plates. No need to pull the wheels just to bleed the system. However, if you don't find a line or hose leaking fluid, you're gonna have to pull the wheels to inspect your W cyl's. I'm betting that is where you will find your leak/s. You can bleed the air out of the system to regain braking before pulling the wheels. However, the safest approach would be to correct whatever is causing the leak/s. Personally, I'm not going very far in any car that has your cars symptoms - at least until I know exactly the problem/s.

One interesting fact would involve how rapidly you lost your pedal. Was it quick or over many miles? This would indicate how bad your leak is.

FWIW, if you find fluid has gotten on your brake linings (and the linings are otherwise OK) there is no need to replace the linings. They can be cleaned very thouroughly with denatured alcohol and a cloth (changea the cloth frequently), wipe 'em dry and then scrub the linings well with a stiff wire brush. If you do this well, your linings will be OK. the thouroughly. (Learned this in my youth and couldn't afford new linings.) Of course, clean and dry the drums thouroughly.

As I said earlier, others on this Forum can advise you far better than I.

Good luck -

Ugh. This is becoming so much more complicated than it really is...Unscrew the cap ( borrow a socket from your neighbor if you have to ) Be careful not to knock any dirt in to the master cylinder...Use a funnel to top-off with fresh fluid...Screw the cap back on...Look around just in case you do have a leak from a wheel cylinder, but you would probably have noticed it by now...Everything look okay...? Good, your done...Go for your 15 minute drive and forget it...Next time ( before another twenty years pass ) check it, say once a year...Good luck

One more thing...make sure the HAND BRAKE is working properly ! This is a single sym,there isn't any secondary sym to rely on in case of a failure. Just sayin..

The socket size you will need is 1 inch. You do need to take this very seriously. As Wawuzit points out you always need a well working emergency brake system to be safe. A lot can go wrong with an over 70 year old car. This could be a wake up call for you to do a complete inspection and maintenance update of the brake system. Please do not start assuming things make sure of them. I will post more info later. The technical info posted above should get you thinking. One the first things you need to figure out is where did the fluid leak out of the system and when was the brake system last serviced? Take your time, Mike

My '38 uses a 15/16" socket if I remember right but my rememberer sometimes fails me. Could be the '40 is slightly larger. I'll bet the next cup of coffee that between Mike and me, one of us is right.

When trying to diagnosis the condition of your brake system I would start by contacting the previous owner. You need to know what he had serviced. For me the brake system is the most critical of all systems on the car. I would also read the manual on your brake system. If you don't have a manual one is available on line at http://chevy.oldcarmanualproject.com/. Next I would go through your past experience with the brakes to remember how they worked when you used them. Did the car pull to the right or left, was the peddle soft, was the car easy to stop? These would hint at the overall condition and hint at potential problems.

Then I would do a visual inspection of everything related to your brakes. You will need to put the car on a hoist or 4 very dependable jack stands for your inspection. I use heavy duty truck stands. You want to do a visual inspection of every inch of the brake lines looking for corrosion and leaks. Check for cracking on the rubber hoses and that all the connections are dry and not corroded. Remember that break lines also rust from the inside out so seeing corrosion on the outside means potential trouble. If you think these may be original lines I would strongly recommend replacing them.

You will also need to remove the tires and brake drums and inspect each brake cylinder for leaks. This is a typical problem area. If you find a leaking brake cylinder I would replace all the brake cylinders. They can be rebuilt but I would trust new ones the most. You need to visually inspect the brake shoes for wear and contamination. Inspect the emergency brake system. Remember you can not apply the brakes including the emergency brakes with the drums off. Doing so causes the cylinders to leak and a mess.

At this point do an inspection of the master cylinder for problems. Your picture shows a stretched brake return spring which I would replace. The master cylinder should likewise show no leaks around the brake line. Do fill it up and follow the above posts. I would purge the system of any remaining fluid and make sure all the bleeders on the cylinders open and close easily. The auto stores do sell a bleeder which works well(no mess and only one person is needed to bleed the brakes).

All the parts for rebuilding brakes are available from Chevs of the 40's and the Filling Station. Anything that looks corroded I would replace. Some parts like the flange plates, anchor plates, links can be sand blasted and repainted. Don't be surprised at the cost of the parts. Doing the work yourself could cost well over $500.00, but how much do you value your safety and your car?

This and the above posts should get you started, as in most projects there is still a lot of detail that has not been mentioned. Keep posting your questions, use your manual, and even consider joining the VCCA which oversees and funds this great chat site. Good luck, Mike

P.S. I checked the 4 master cylinder filler plugs I have and they all take 1 inch sockets. The master cylinder for the years 1940 to 52 are all suppose to be the same. So I would start by purchasing a 1 inch socket. If you want to do any front suspension work you will need sockets up to 1 1/8 inches.

Just because the fluid is low doesn't necessarally mean you have a fluid leak...In fact, most of the time you don't...It's merrily caused by wear on the brake pads...As the pads wear and are moved closer to the drum the fluid level naturally drops over time...

You could try contacting the previous owner...If you don't own a shovel to dig him up you can probably borrow one of those, too.

Like I said before...Add some fluid, put the cap back on...Press the pedal a few times slowly and voila, the pedal should return to normal...With nothing else to worry about...

This is not something to lose sleep over ( hint, hint )

If your parking brake doesn't lock-up the rear wheels on a hill or you have to apply them fully to work...You need to get under the car and adjust them...

Tell us how it goes...Good luck and god's speed, although you shouldn't need either...Kevin

I bought a 1" socket a few minutes ago and had to use a pry bar to get the cap to move. Once it moved a little I was able to take the cap off with my fingers the rest of the way. No fluid in the master cylinder.

I added some fluid, put the cap on, pressed the peddal but only 1/3-1/2 way down as Gaither recommended. I did that a few times but the peddal doesn't have any resistance. How many times should I need to push it down?

I'm going to do it a few more times and check the fluid level again. So far there is no leaks that I see on the ground or by the tires other than the few oil drips.

If you keep doing that eventually the brakes will start working or you're going to see fluid comimng from the rubber hoses or dripping out from behind the brake drums or the master cyl will show a leak. Something will show up.Does the rubber around the Master cyl. look rotted or cracked?

Woot! I took the cap off and press about half way slowly and I saw air bubbles. I did this maybe 30-40 times and by then there was no more air bubbles and the break would only press down maybe 1/3 of the way and it took some force to push it down, yay!.

Now the breaks work way better than they ever did previously. I'll keep an eye to see where it drips but so far I don't see anything. Is it possible that it will only leak when it's driving and breaking and that the fluid burns off. Meaning no "spot" on the garage floor.

I just want to comment on the one of the posts. As the shoes wear and they are adjusted, the hydraulic cups and pistons return to the exact same spot and thus there is no change in fluid volume in the system.

If a wheel cyl. is leaking, it will show on the inside of your tire. If the car sets for some time, look at all four and it will tell you witch one is leaking.

If the pedal goes down to a third of the way, it just means you need to go around and adjust the the pads out to where they should be...Then check the master cylinder fluid level again, as it will probably have dropped some...

Adjusting the pads are more than likely gonna be a pain 'cause they haven't been adjusted in 20-30 years...And the adjusting caps will be frozen and you'll just tear the "stars" off and get nowhere...

The bubbles you see are probably just the air in the lines floating back up through the lines...

If'n it's working well enough for now, I'd just let it go...Because once you find that the adjusting caps are stuck, you'll realize your going to have to tear it all down and repair the wheel cylinders before they can be adjusted...Didn't you have something else you wanted to do next weekend...And the weekend after that...?

Now why would the fluid level in the master drop after adjusting the shoes?

My mistake...Fluid level should remain good...I was confusing it w/disc up front...My bad

Obviously, I'm not a brake tech. However, I have never had to add fluid unless I had a leak. Further, if one has to add fluid, there is at least one leak (likely more) in the system.

The leak may be such that one can ignore repairing it for some measure of time. It will continue to leak until whatever it is gets repaired. IMHO, it is never reasonable to ignore a brake problem. It just might get you killed!

FWIW, you can run drum brakes until they are metal to metal and still not run the M cyl dry. The same is true of disc brakes. The M cyl fluid reserve is calculated to prevent having to add fluid even if you do run linings down to metal to metal. However, the same cannot be said of a fluid leak in the system.

PCM2A, please keep us updated on your brake issue. I think you have a problem somewhere. Hopefully, you can solve it before it causes an even bigger one. Good luck, Mike

P.S. It is obvious from your previous post that the master cylinder fluid cap has not been off in a long time, which to me indicates the brake system has not been serviced in a long time. While you are messing with the master cylinder you might as well grease where the brake and clutch peddles are mounted to the master cylinder. Just remove the screw or grease zit and fill the cavity.

The pistons in these old Huck brakes return to the same spot no matter how much wear there is on the shoes. It doesn't affect the fluid volume sitting in the system.

Disc brake caliper pistons gradually move their rest position as the pads wear. The volume in the master cylinder does change as a result. I know of at least one modern vehicle that I have owned where fluid level in the master will initiate a warning light when the pads are worn significantly. I assume this is probably a very common design feature today.

216, True, but that warning light was to tell you the brakes needed attention - not that the M cyl was dry.