Under floor Master Cylinder Brake Bleeding
 

Stupid thread but I've never bled brakes with an under-floor master cylinder. Looks like the only way to do it is to pull up the floor mat, taking the risk of breaking it into pieces, and remove the floorboard panel to access the master cylinder.


On a previous thread, saw a recommendation from Tubman and others to use speed bleeders. They're on the way from Speedway motors. Any other tips to make this less hard than it needs to be?


I've bled brakes before so the concept of getting air out of the brake lines isn't news.


Can't believe my Columbia rear end project is coming up on it's one-year anniversary. All the things I've had to learn have been very time-consuming. Thank goodness for the San Diego chapter of the Early Ford V8 club for expertise and tool loans.

Re: Under floor Master Cylinder Brake Bleeding
 

You should not have to remove the floor board . Theres an access hole above the master . Getting the master cap removed can be a task in itself,if the master is old and hasn’t been topped off recently.
I bleed them by myself but having a helper makes it easier. I’ve never tried the speed bleeders .
My method is simple . Loosen the furthest bleeder a very little , add a flexible tube to the bleeder ( must be a snug fit ) have it end in a clean jar half full with new brake fluid.
Cut a piece of wood or pvc to fit snugly between the seat and brake pedal . Pump or push the brake pedal , don’t let it up , insert the wood ( adjust seat if needed ) , go back and tighten the bleeder . I forgot to mention the hose should near the bottom of the jar and pretty secure.
I usually do it 2-3 times for each wheel .
Good luck .
PS I’ve got a matching 41 coupe .

Re: Under floor Master Cylinder Brake Bleeding
 

I bought this, I haven’t used it on my ‘41 yet, but I have used it on the Corvette I use to have and my truck.

Re: Under floor Master Cylinder Brake Bleeding
 

Changing the master cylinder on my 50 Ford tomorrow. Going to a mechanic friend of mine. There will be two of us and he has a lift which I think will make it much easier. Mine has been leaking a little and I just add a little fluid. That cap is hard to get on and off. Hard to get started straight. Mine is plastic and I am afraid I am going to cross thread it. The new one is metal. Hope we can get it on without too much trouble. May take on the motor and transmission mounts if the master cylinder goes ok while it's on the rack.

Re: Under floor Master Cylinder Brake Bleeding
 

Be aware that the caps on some of the new - or rebuilt - MC units are nearly impossible to remove once installed. They can also make adding fluid to the reservior a sputter of curses to the designers.

Simply reusing the older cap worked much better - and improved my vocabulary.

Re: Under floor Master Cylinder Brake Bleeding
 

FortyNiner

IF it weren't for old car/car mechanic's in general...We might could be preachers!!!! Lol

Re: Under floor Master Cylinder Brake Bleeding
 

Be sure to bench-bleed the master cylinder before installation (some come that way) and adjust your brake pedal play as necessary. My '47 has an access hole in the floor pan - just need to lift the carpet. I have Speed Bleaders and like that they allow one person to bleed the system. Regarding brake fluid, over time DOT 3 and 4 brake fluid absorb moisture which can damage the master and wheel cylinders. I have not converted the system for use of low-absorption DOT 5 because of concerns its more easily may "leak". So I have my brake fluid changed every three years. For my '47 the shops can remove the master cylinder cap from beneath the car. Then they open all the wheel cylinders and suction the old fluid out. They pump new fluid in from below. The cost is minimal given the benefits.

Re: Under floor Master Cylinder Brake Bleeding
 

You'll like the speed bleeders. I used them on all my projects. And, if I'm replacing everything but the hard lines, I'll switc to Dot 5 and never have to change fluid again. And, you can use the speed bleeder and then unscrew them and immediately replace them with the original bleeders and use them for another project.


I have never bench bled a master cylinder because I don't know how to do it. I will read up on it before putting the master cylinder in my 52 Willys M38A1 jeep.

Re: Under floor Master Cylinder Brake Bleeding
 

One man -I use a vacuum suction bleeder 'friends' vanish when work is involved. Newc

Re: Under floor Master Cylinder Brake Bleeding
 

Are most of us away from DOT 3??

Re: Under floor Master Cylinder Brake Bleeding
 

I put DOT-5 in a '68 Corvette in the late nineties. It was a bitch to get bled properly. It had to be heated to drive out the air and I had to close the bleeder after each pump of the pedal (this almost cost me the relationship with my late girlfriend). After I finally got it in with a firm pedal, it was great. However, it was such a problem that I swore off it after then.

These days, I use DOT-4 in all of my cars. I have an electronic doohickey I got from Amazon that tests the moisture content of the brake fluid. I have found that collector cars stored under good conditions will go 9-15 years before the moisture content gets high enough to become a potential problem. I test my cars every couple of years, and if the moisture content starts getting to be a problem, I grab a fresh bottle of DOT-4 and run it through the system,. It has been mentioned that I am a believer in "Speed-Bleeders". That is true, and allows me to cycle the fluid with a minimal expenditure of time and effort all by myself.

I use DOT-4 instead of DOT-3 because it's a little better and costs about the same. Bleeding is the same as DOT-3. If you have the extra money, a lot of time, and a cooperative companion, DOT-5 is OK, but I'm short on all of those these days.

Re: Under floor Master Cylinder Brake Bleeding
 

Like was mentioned, there is an access in the floor. And yes the fine threads on the cap will test one's patience. I was topping it off every couple days until I installed the new M/C on my '50 this summer. I WAS using my wife for the "pedal pusher" for bleeding. Switched to the mityvac and we're still married.

Re: Under floor Master Cylinder Brake Bleeding
 

install a remote fuller

Re: Under floor Master Cylinder Brake Bleeding
 

This is required for dual bowl master cylinders but not required and a waste of effort for the original single bowl masters.

Re: Under floor Master Cylinder Brake Bleeding
 

Keeping the master cylinder full while bleeding was one of the biggest challenges with the MC under the floor, especially if you're working alone. This kit worked reasonable well and wasn't expensive.

https://www.harborfreight.com/brake-...der-92924.html

Re: Under floor Master Cylinder Brake Bleeding
 

Clem: DOT 3 and its later version DOT 4 are about the same. I probably gravitate to DOT 4 but have seen both on store shelves. Either can absorb moisture that seeps into the system. I paid only $40 to have a shop replace my fluid - saved me a lot of time, work and trouble. To the good, the mechanic got a chance to check my system over.

Re: Under floor Master Cylinder Brake Bleeding
 

Floor mat was so brittle, couldn't help but tear it in a couple of places getting to the access hole. Even though I was very careful, the access cover crumbled when I removed it. I see that C&G Ford parts has another in stock. As usual, I'll pay more in shipping than the minor part's purchase price. But it's worth it not to have to make a 90 minute round trip to Escondido.


Tested the brake fluid after the first pump and much to my surprise, it appears to be DOT 5 rather than the DOT 3 I'd purchased to do the job. Glad I tested it on the bench rather than in the car. One more trip to the auto parts store for a bottle of DOT 5. At first it looked they didn't even have any. They had about 20 product selections: 15 DOT 3, 3 DOT 4, 1 DOT 5.1 and one DOT 5. Took some time on my phone trying to figure out if DOT 5.1 is compatible with DOT 5 before finding the single DOT 5 product. By the way, no. 5.1 isn't compatible with 5.

Re: Under floor Master Cylinder Brake Bleeding
 

DOT5 is getting hard to find. It seems it is not compatible with ABS brakes, so the great majority of the contemporary manufacturers don't use it. It is used mainly by the military and in a few high-zoot specialty applications. It will only get harder to find and more expensive.

It's too hard for me to bleed anyway, so I gave up on it 15 years ago.

Re: Under floor Master Cylinder Brake Bleeding
 

How do you test brake fluid to determine if it is dot 5?

Re: Under floor Master Cylinder Brake Bleeding
 

Poured some of what I bled out of the system into a deli container lid then poured some new DOT 3 fluid from the auto parts store into it. They did not mix, like oil and water. Then poured some water into it. The water mixed with the new DOT 3 but not the old stuff. Couldn't say that proves 100% the old stuff is DOT 5 but those would be the symptoms.