1950 Ford Custom Brake Master Cylinder

[DNLs1930]

Is the stock OEM 1950 master cylinder reservoir "pressurized" during brake application or pedal return?


A little history on the issue and reason for the question.


I had a soft pedal and checked the reservoir (once I got the cap off which I believe has been on the car 75 years too) which was low; no apparent leaks. This is the second time for a soft pedal in the last two years. The cap and gasket were very dirty.


What I think was happening is a little fluid was escaping from the master cylinder cap each brake actuation and the residual was moving around under the car as it was driven. No static leaks other than oil that probably had some brake fluid in it looking back...




I applied the brakes a few times; I had a soft pedal due to low fluid and after adding fluid the pedal firmed up nicely.


My question is: Is the master cylinder reservoir pressurized? Because while I was applying the brakes with the cap off fluid would "squirt up" an inch or so in the reservoir which makes me think the cap is holding pressure but probably not much but enough to make the cap leak if the seal is bad.


Pretty sure the cap and seal are decades old. I cleaned the threads out they were very dirty and cleaned the gasket area which was caked with gunk. I reinstalled the gasket and added an o ring to seal it which seemed to work very well.


I will bleed the brakes just to make sure there is no air in the system and change out the fluid.


THANKS IN ADVANCE for any thoughts on this.

[glennpm]

Originally Posted by DNLs1930

My question is: Is the master cylinder reservoir pressurized? Because while I was applying the brakes with the cap off fluid would "squirt up" an inch or so in the reservoir which makes me think the cap is holding pressure but probably not much but enough to make the cap leak if the seal is bad.
THANKS IN ADVANCE for any thoughts on this.

No, the master cylinder chamber is not pressurized. The "squirt" that you see is the pressurized fluid outboard of the master cylinder, the brake lines and wheel cylinders, squirting out the bypass port when the brake pedal is released. In addition, the master cylinder caps and gasket are vented to maintain atmospheric pressure. I just looked at a few links and I liked this one for graphic explanation.

https://www.youtube.com/watch?v=rcvhntRbTb4

[DNLs1930]

Originally Posted by glennpm

No, the master cylinder chamber is not pressurized. The "squirt" that you see is the pressurized fluid outboard of the master cylinder, the brake lines and wheel cylinders, squirting out the bypass port when the brake pedal is released. In addition, the master cylinder caps and gasket are vented to maintain atmospheric pressure. I just looked at a few links and I liked this one for graphic explanation.

https://www.youtube.com/watch?v=rcvhntRbTb4

Thanks for the link to a really good video on the operation.


So the return bypass fluid would increase the pressure in the reservoir but the cap it is vented so it can't build up; makes sense.


I will keep checking the fluid level and see what happens with the new seal on the cap...

[TJ]

When is the last time you did an R and R on the brake system? If the cap and seal is decades old I suspect the system is decades old. It's time to replace all the components and flush the lines of old fluid.

[glennpm]

Originally Posted by DNLs1930

Thanks for the link to a really good video on the operation.

So the return bypass fluid would increase the pressure in the reservoir but the cap it is vented so it can't build up; makes sense.

I will keep checking the fluid level and see what happens with the new seal on the cap...

Yes that's correct the cap vent prevents pressure buildup from the pressurized brake fluid squirts out the bypass hole.

[leon bee]

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I'm with TJ. I love fresh brakes.

[glennpm]

Yes, every two or three years, you should flush the whole system. Look into your master cylinder reservoir and when it starts looking brown, it's time to do it.

[DNLs1930]

Originally Posted by TJ

When is the last time you did an R and R on the brake system? If the cap and seal is decades old I suspect the system is decades old. It's time to replace all the components and flush the lines of old fluid.

Well that's a good question...
Best answer is: A full system fluid change will be done soon.
Thoughts of changing over to a dual master cylinder is on the list too... just need to look at all the modifications needed for that before opening that can o worms...

[Flathead Fever]

I would definitely flush all the old brake fluid out. On a vehicle that is driven regularly you are supposed to replace the fluid every two-years because the brake fluid attracts moisture. I was a fleet mechanic for 30-years. What we did was drain the master cylinder and fill it with new fluid. Then as we were changing the brakes we would open up the cylinder on that wheel and let the old fluid gravity drain out while we were changing the lining. I would look at the rear of that master cylinder and make sure it's not leaking from the rear seal. It's normal for the brake fluid to squirt up as the pedal returns, that's just the fluid returning back to the resevoir, If you have a bad gasket it can slosh out and empty your resevoir You want to really keep an eye on single systems, you hace no emergency secondary resevoir to stop it with like modern cars There's nothing wrong with a single systems "if" they are maintain.

[DNLs1930]

Originally Posted by Flathead Fever

I would definitely flush all the old brake fluid out. On a vehicle that is driven regularly you are supposed to replace the fluid every two-years because the brake fluid attracts moisture. I was a fleet mechanic for 30-years. What we did was drain the master cylinder and fill it with new fluid. Then as we were changing the brakes we would open up the cylinder on that wheel and let the old fluid gravity drain out while we were changing the lining. I would look at the rear of that master cylinder and make sure it's not leaking from the rear seal. It's normal for the brake fluid to squirt up as the pedal returns, that's just the fluid returning back to the resevoir, If you have a bad gasket it can slosh out and empty your resevoir You want to really keep an eye on single systems, you hace no emergency secondary resevoir to stop it with like modern cars There's nothing wrong with a single systems "if" they are maintain.

Ok so found the leak! Pretty sure the fluid is leaking from the master cylinder which is no surprise really. The "oil" under the car was in fact part brake fluid probably coming from the push rod. I haven't removed all the mud shield to verify but it will be removed when the Master cylinder is replaced.


After reading the replies here I decided to replace the entire system minus the hard lines. I purchased the ShoeBox Central "kit" with all the wheel cylinders, hoses, linings and hardware so the car should be good to go brake wise for a while.


ANY TIPS on a full system replacement? My plan is to open all the lines bleed all the old fluid out cap them off to prevent contamination, replace the master cylinder and each wheel hose, cylinder linings and hardware. THEN bleed the entire system with the new master cylinder installed...


Any issues you guys can see? Different sequence maybe?


TIA

[glennpm]

Originally Posted by DNLs1930

After reading the replies here I decided to replace the entire system minus the hard lines. I purchased the ShoeBox Central "kit" with all the wheel cylinders, hoses, linings and hardware so the car should be good to go brake wise for a while.

ANY TIPS on a full system replacement? My plan is to open all the lines bleed all the old fluid out cap them off to prevent contamination, replace the master cylinder and each wheel hose, cylinder linings and hardware. THEN bleed the entire system with the new master cylinder installed...
TIA

Bleed the brake lines after you install the new master. This will give you nice clean fluid throughout.

I like to use 1-1/2 turns of yellow teflon tape, wrapped CW, which is good with hydrocarbons.

Look at the new wheel cylinders carefully and keep the old until you know they are working correctly. Some of the "new" wheel cylinders have issues with bleeder drilling and cups being too short ...

https://www.fordbarn.com/forum/showt...rake&showall=1

[Kens 36]

Your plan looks sound to me.

One thing I recommend to make your fill/flush easier is the Motive Products power bleeder. https://www.motiveproducts.com/collections/all

Super nice for a one-person job. Make sure you get the correct adapter for our master cylinders. I also like the quick release fittings connector. PM if you have questions.

Ken

[glennpm]

Originally Posted by Kens 36

Your plan looks sound to me.

One thing I recommend to make your fill/flush easier is the Motive Products power bleeder. https://www.motiveproducts.com/collections/all

Super nice for a one-person job. Make sure you get the correct adapter for our master cylinders. I also like the quick release fittings connector. PM if you have questions.

Ken

I just bought and used the attached which functions based on low pressure from a venturi. I usually use a vacuum pump but have also pressure bled. This tool I like the best.

https://www.amazon.com/dp/B0CDV3D5ZS

[DNLs1930]

Ok so probably NOT MY LAST QUESTION...


With the planned replacement of the master cylinder, all flexible brake hoses and wheel cylinders what would be the brake fluid of choice?


DOT3 seems to be the front runner based on what I have read so far...

[glennpm]

I like DOT4 with it's higher boiling point. Be ready for the DOT5 comments :-)

[tubman]

Actually, this thread got me thinking and I did a little research. I used to be a "DOT4 guy", but after a little digging, I'm starting to lean towards DOT3. The main advantage of DOT4 fluid is a higher boiling point (around 440° F as I recall). However, its downside is that it picks up moisture more quickly than DOT3. The boiling point of DOT3 is 407°. The way these cars are used and the very nature of the brake systems tells me that we will probably never need those extra 30°.

As things stand now, I think I'd rather have the reduced moisture pickup and retention than the extra protection offered for "heavier vehicles and trailer pulling".

Speaking of moisture in brake fluid, I purchased a tester about 15 years ago that will determine the moisture content of your DOT3 or DOT4 brake fluid. I use this every year to check my vehicles. I have found that for collector cars stored under favorable conditions, DOT4 brake fluid doesn't reach a minimum moisture level until after 9 years. If DOT3 extends that even a year or two, I'll start using it, because I don't use my old cars to pull trailers and I hate changing brake fluid.

I put DOT5 in a '68 Corvette I owned years ago. After I got it in and bled, it was great, but not near worth the effort.

[glennpm]

Thanks, I didn't know about the longer service life for DOT 3. I have motorcycles that specify DOT 4 so I'll be staying with DOT 4.

I used DOT 5 in my '40 decades ago but it never felt solid regardless of multiple bleedings. I lived with it for a while and then flushed with DOT 3 at that time.

Now I'll see what my unused tester that I bought a few years ago :-) tells me tomorrow.

[Kens 36]

Good info guys. I've always been a DOT 3 guy, mostly because we have used it for eons. I knew some guys that got on the DOT 5 bandwagon and regretted it, mostly because of the soft pedal. Just never went to DOT 4.

I'm amazed to hear that some conventional fluids could laxt that long, even though I have often been delinquent in flushing. Will be interesting to see your results.

[DNLs1930]

Thanks for the reassurance 4 has to be better than 3 right and then there is 5

DOT 3 for the win... Parts arrive today so should be fun after watching a few videos on the replacement well not sure fun was any of the words ised to describe the process

[Kens 36]

The process is tedious, but rewarding. I recently replaced the master cylinder on my '50 convertible, which is made much more difficult by the additional X frame.

The YouTube video by Chris at Shoebox Ford is a very good guide. Make sure you take pictures of everything you take apart to help you get them back together. There are lots of washers and spacers in place that may be glossed over in the video. Avoid taking any shortcuts.

If you hit any snags, I'll be glad to help you.

Ken