No restistance at the brake pedal: Master cylinder worn?

[50s-coronet]

Hi all and happy easter!

I noticed (fortunately in my garage!) that the resistance of the brake pedal of my 1950 dodge coronet was drastically reduced until I could touch the floor with the pedal. My first guess was that there might be an issue with the master cylinder. I took some photos from below:

 

 

 

In the 2nd picture it seems that there is a leakage of the master cylinder. I have never done an overhaul of the master cylinder before, so I'd appreciate any information on it. Is it something I can do in my garage? In the workshop manual they wrote that for bleeding I need a special tool.

 

I also found on Andy Bernbaum's an offer for a master cylinder kit (Part # G142), which might be required. Any experience/recommendations on it?

 

Thanks in advance!

[TodFitch]

How long since the car was driven? Reason I ask is there is a residual pressure valve in the master cylinder that is supposed to keep just a little pressure in the system so that the cups all seal. If it sits for a long time you can lose that pressure and then get some air into the system. This is especially true if one of more wheel cylinders are pitted due to internal rusting.

 

I'd do a full inspection of all cylinders and joints (brass fittings, hose connections, etc.) and see if there is any indication of leaks. Fix any leaks found, then fully bleed the system. Once it is bled, you can apply some pressure and then recheck for leaks. There is a reasonably good chance that the issue is not in the master cylinder and you'll want to find out where it is rather than throw time and money into an arbitrary part based on a guess.

[soth122003]

Agreed. Check the wheel cylinder and lines first. Just replaced a wheel cylinder on mine yesterday. As far as the MC goes the special tool is for bench bleeding  and it makes life easier when bleeding the whole system. When I do an MC I pull it apart hone the cylinder wall and when I put it back together, put the MC in a vice, cap the ports and fill with fluid. Then using short 1-2 inch strokes on the pedal lightly pump the MC till no air is coming out in the reservoir. This gets rid of 99% of the air and the rest will bleed out with the main bleed of the system. This vid should help with a lot of your questions.

 

Joe Lee

 

 

[kencombs]

Bench bleeding works best if you attach little test lines to the port and the other end in the reservoir. Below the fluid level. Pump unto no bubbles then cap the ports. 

[Frank Elder]

Be a Troubleshooter not a Parts changer........I heard that sage bit of advice from every Aircraft Maintenance Master Chief I worked for. 

[Tom Skinner]

Amen Frank, Bleed them brakes two or three times around after checking for leaks at Wheel Cylinders. You should have a pedal by then or then maybe its the MC

[50s-coronet]

Oh, the Master TEC! How could I forget about him!

 

@TodFitch: indeed the car was standing for about five or six months. But when I drove it the first time, the brakes still worked fine (although the travel of the pedal was already a little longer). The second time then there was no more resistance.

 

So this weekend I will start to have a look at the wheel cylinders. Since the pedal could be pushed down to the floor, I assume it's a leakage. And no parts are ordered. Just some brake fluid might be required...

 

Thank you all so far for your advices! I will give an update soon with some pics again.

 

Regards

[desoto1939]

Before adding just brake fluid you will need to know what type of brake fluid was used prior to you getting the car, Dot3, Dot4 or Dot5 which is synthetic.  Dot 5 can not be mixed with the other types.  If you are converting to Dot5 then you need to flush the entire brake lines  with denatured alchohol, replace all rubber lines and rebuild the brake cylinder and also master cylinder.3

 

So do some research prior to putting in brake fluid.

 

Rich Hartung

desoto1939@aol.com

[50s-coronet]

lot of time passed by in the meantime (I had to move and new job etc)...

I've taken out the master cylinder because it was the only suspicious one and disassembled it... or let's say the parts popped out. the rubber parts were worn and I ordered new ones.

 

the only thing is, that the kit is not identic to the drafting in the shop manual. So the question is, where the correct placement of the rubber ring is. I assume it's between the piston and the cup. can anybody tell me? (this rubber ring has already been in the "old" assembly, maybe it also was not original; however, the ring is required to get the correct pedal way.

[keithb7]

Hi the same rubber ring threw me for a loop too. It is the seal for the 

Valve .Seen here. It goes in first. In front of the valve  

 

[keithb7]

This diagram here from my Motors Manual shows it. 

 

 

[dale]

By the looks of the master cyl and lines Id suggest you replace the whole system with new parts.. It IS your life on the line.

[50s-coronet]

@keithb7, I think your post did save me some two hours and a lot of nerves! (; many thanks! !

 

So parts were assembled, system was bled and flushed with fresh brake fluid.

 

Afterwards I took the vehicle to a few test rides. The result was absolutely satisfying with regards to the performance of the brakes.

 

After some 50 miles I took a look into the reservoir of the master cylinder and saw that the filling line was still at the level like before.

 

The plan is to monitor the master cylinder more regularly. Would you agree to this strategy, @dale?

[Frank Elder]

Doggone I like it when somebody posts the solution/how it turned out to their problem!

Good job 50s-Coronet!

Edited July 4, 2019 by Frank Elder

[keithb7]

With these old cars, it is imperative that the master cylinder fluid be checked regularly in my opinion. A single reservoir master cylinder,  and other old parts leaves little room for error.

Maybe I am paranoid, but it is my life on the line. I check the master cylinder fluid level often. I look for signs of leakage at all the wheel cylinder and at all brake line connections.  I pull my drums off at least a couple of times each cruising season to inspect my breaks.  Make sure your parking break is well adjusted and functioning properly. If may be your last life line. It's your reserve chute. 

[JSabah]

im a bit confused too.  I took some pictures of my master cylinder as I took it apart and had a seal on the push rod (see pic below).  but from what I read above I think it may have been wrong and that seal goes in front of the valve.  and the groove that my seal was found in is actually for the dust boot.  Can someone please confirm - it would make sense as the old valve did have a seal on it (as part of the spring valve assembly) 

 

[JSabah]

So final assembly is supposed to look like this?

 

can you confirm that the valve assembly is in the right direction.... also as long as it is all apart (other than honing the cylinder) how would you remove rust from the reservoir? Lacquer thinner followed by acetone, muriatic acid? Obviously before I put it all together 

[keithb7]

This next pic is missing the valve seal, which is shown above in my other pic. However this pic does a good job showing placement of the rest of all the parts:

 

 

Edited July 12, 2019 by keithb7

[keithb7]

3 hours ago, JSabah said:

im a bit confused too.  I took some pictures of my master cylinder as I took it apart and had a seal on the push rod (see pic below).  but from what I read above I think it may have been wrong and that seal goes in front of the valve.  and the groove that my seal was found in is actually for the dust boot.  Can someone please confirm - it would make sense as the old valve did have a seal on it (as part of the spring valve assembly) 

 

 

 

The groove in your pushrod shaft is for the dust boot.

 

There certainly is no lack if confusion around how these parts go together.I understand your guys' thoughts. I too was scratching my head... When I took my first ever old Mopar master cylinder apart it was screwed up and assembled wrong too. By the previous grease monkey in there. See this pic below. Look when my valve seal is. On the push rod beside the cylinder stop. LOL. Then on top of that you get old photos that don't show the valve seal. Guys get to guessing where to put the extra parts found in the reseal kit.

 

 

Edited July 12, 2019 by keithb7

[westaus29]

JSabah - keithb7 is correct. The ring seal goes infront of the valve. The groove in the push rod is for the boot. I recently helped a friend where the ring seal was in the wrong place AFTER the valve. For some reason it had worked ok until he changed rubbers on the rear wheel cylinders, then he could not get the master cylinder to pump to bleed the brakes.

Do NOT use muriatic acid on car parts (exception maybe a very clogged block for rebuild). This acid is way too strong, attacks the metal faster than the rust and will penetrate into the metal and is very hard to neutralise. I recently used it on a rusty manifold and it is still sweating rusty acid - a lesson learned, be patient!. Dilute phosphoric acid is better as it coats the metal with ferric phosphate which passivates it. It is the main agent in most rust removal preps that you can get from a hardware store. Citric acid is more gentle (lemon juice) but a bit slow. If you can find an acid based radiator cleaner that is labelled as sulfamic acid (looks like sugar), it is very gentle and fast and I believe contains an inhibitor so it doesnt attack the metal. I have used that on instrument mechanisms without damage. Not easy to find.

Flush with water then methylated spirits (not sure what you call it - methyl alcohol as sold at hardware) Jim

Edited July 12, 2019 by westaus29
addition

[kencombs]

Vinegar., plains ole white vinegar is my go rust remover. Soak. Scrub with hot water and brush immediately coat clean surface with brake fluid

[westaus29]

You are right, I forgot vinegar. It is slower and gentler than lemon juice but for me lemon juice is free as have a tree that is always laden. At one stage diluted molasses from a stock feed outlet was in vogue, but I found it also is agressive to the metal and the smell got the no vote. I would always recommend a final rinsing with methylated spirits (you call it denatured alcohol, available in hardware stores?) before brake fluid as it dissolves any remaining water then quickly evaporates. I always have a squirt bottle of it handy for brake work as it also washes off brake fluid.