Greetings,

First time posting so hoping this is the right forum. I have a low-mileage (~70K), manual transmission, mostly original parts 1976 F100 that I've had for 3 years and drive a few times a month (<500 miles/year). The only work I've had done is replace the points distributor with an HEI.

About a month ago, I was on my to an appointment when the brakes sank to the floor within 2 blocks of leaving the house. Luckily, I was next to my neighborhood mechanic so I dropped it off and caught a ride to my appointment. Mechanic said the master cylinder was empty so he topped it off - no charge. I didn't bother investigating further and my brakes worked fine the next time I drove it a week later. By the way, I don't have a brake booster.

After letting the truck sit for a couple weeks, I tried to take it for a spin but the brakes sank to the floor again. I checked for leaks (master cylinder connections, lines, front and back wheels, along the firewall) but didn't find anything right away. I did notice that there was a lot of gunk along the top of the reservoir and gasket so I cleaned both.

Yesterday, I refilled the reservoir with new fluid, pumped the heck out of the brakes and the fluid came back dirty (see photo). Today, I finally located the leak - at the firewall, mostly under the master cylinder but also inside the cab at the pushrod.

Question 1: Should the first step be to replace the master cylinder?

This is my first old vehicle and I don't have a ton of DIY experience but replacing the MC seems pretty straightforward.

Question 2: Would the next step be to have a brake shop flush and bleed the brake lines and check the pads?

 

Yes replace the master cylinder. Before installing new one make double sure it matches the old one. Typically its best to bench bleed the MC before install. Yes absolutely if you are not sure how to do it, have a shop bleed all 4 wheels to flush out old fluid and air. Problem is if you replace the master at home, then you have to drive it to the shop without brakes or very poor brakes. Worth it to have the brake system gone over front to back. If the rubber flex hoses are old they should be replaced now. One at each front wheel and one that drops down to rear axle.

 

Ok so no booster, only mean NO power brakes. IMO the master cylinder seal is shot, I am sure you can get a rebuild kit. Faster and easier to just get a replacement. Read up on how to "bench bleed" a master cylinder prior to installation.

To get that old nasty brake fluid out, you are going to need to drain and then best to flush the system. Meaning drain all the old fluid out the lines via the wheel cylinders. And there are 4 of those, one at each wheel.

 

I would get a new MC, bench bleed it , put it on and bleed your brakes until the fluid looks decent.

I wouldn't worry about overdoing it because it's almost certain your wheel cylinders and calipers are not long for the world, so you can finish the job whenever you replace them.

Your system looks like this because DOT 3 attracts water, that's why I use DOT 5 in my classics it repels water. I won't destroy paint either.

 

Might talk to your "neighborhood mechanic" ... maybe he'll even appreciate you helping him swap and bleed. Brakes ever been relined? Wheel cylinders are right there. At least three rubber brake hoses to check, four if 4wd. I have used new return / hold down springs, etc, but usually I don't myself.

I'd spring for the pizza too.

 

Notice how rough the top surface where the cap is suppose to seal?
It looks like it is not machined. I've seen new ones like this.
The master cylinder is tilted to the rear and the cap can not seal this rough surface.
So the fluid is always dripping down the firewall.
Moisture enters under the cap and damages the pistons and seals.
I would not buy one that is not smooth on the top edge.

 

Thanks everyone for your advice.

I bought a new Duralast NM1617 MC and one-man brake bleeder kit from Auto Zone today.

Removing the old MC and pushrod wasn’t that difficult. However, I noticed that the new pushrod was slightly shorter than the old one.

After connecting the new pushrod to the pedal, the brake lights stayed on. I figured this was because the brake switch wasn’t being depressed so I simply unplugged the wiring.

Then, I added DOT 3 brake fluid to the new MC and bench bled it using a long Klein Tools square screwdriver. I don’t have a vise so I held it on top of my toolbox. Not ideal but it worked. After pumping for about 10 minutes, I placed the plastic plugs that came with the MC back into the brake line inserts and installed it.

After installing the new MC and inserting the pushrod, I had trouble getting the new rubber push rod boot over the back of the master cylinder but it appears to be on. Then, I connected the pushrod to the pedal and adjusted the brake switch by moving the plastic casing up and using a tie wrap to prevent it from slipping.

I carefully took the truck for a ride around the (hilly) block and the brakes work as good as ever.

I didn’t even have to bleed the wheel cylinders but I plan to take it in and get the brakes flushed soon.

Again, thanks for all of your advice. Let me know if y’all have any questions or additional advice.

 

​​​​​​​This has got to be what was going on. Fortunately the new reservoir was smooth on top and the clamp closes snug tight.

 

I used DOT 3 because that’s what was already in the lines. When I get the brakes flushed, I may switch to DOT 5. Are there any negatives to using DOT 5?

 

Congrats on the repair, not get a new steering shaft flex (rag) joint coupler on order. The break fluid breaks down the rubber over time. I am sure yours is short on life.