Got a new MC not rebuilt, and after bleeding still not getting a firm pedal. Just goes to the floor. Seems to be the rear system because I keep topping it off. I've bled all wheels at least three times. Can't see any fluid at the wheel cylinders or any where along the brake lines so I'm really bumfuzzled as to where the fluid is going.Any ideas?

 

Power or manual brakes? If power, the brake booster could be faulty. I have also read a lot of people need to adjust the push rod on the MC to achieve the correct length for pedal stiffness/brakes being applied.

Also, if you keep having to top off the MC rear brake reservoir that would lead me to believe that you have a leak somewhere. What is the condition of the rear brake wheel cylinders. Those will leak inside of the drum, sometimes making it difficult to see that they are leaking.

 

Sorry, it has Power brakes. Thinking that I should get a firm pedal regardless, right? I'm also suspecting wheel cylinder, but I expect to see some fluid after several applications of the brake pedal and theres the issue..... Don't see any. I replaced the RR cylinder a couple years ago so yeah I should expect the LR to be the culprit now. When the RR failed it was pretty obvious due to the amount of fluid all over. Really hate to pull the rear apart....It's such a PITA dealing with gear oil. ( The rear end is a Dana 70). Power booster is original so I'm pretty sure the push rod is OK. Only thing I can think of is the fluid is leaking into the booster but wouldn't the MC prevent that from happening? Another thought to is I got another bad MC off the shelf. (NAPA part)

 

Did you swap the Dana 70 (used 8 lug 16" wheels) into your F150?

1976/79 F100/150 could have either a Dana 44 or Ford 9 inch.

Dana 70: Some 1961/62 F350; 1963/78 F350; 1979 F350 2WD

 

No ND....I'm working on my F350. Don't have the F150 any more. Unfortunately my username still says differently. The F350 I have was my Dad's truck that he bought new so it's mostly original except for the 4spd trans he swapped in. Some day I plan to put the C6 back in.

 

If you have to keep adding fluid, it's going somewhere! Dana 70s will leak inside the drum (bummer) or the Master Cylinder can leak into the booster. (Bigger bummer). Otherwise, you should be able to find the puddle

 

When changing out MCs on a power brake booster, never assume that the replacement MC will be just fine if it's bolted on without measuring the depth of the MC bore to the length of the booster output rod --even if the replacement MC is supposed to be a direct OEM replacement. Machining tolerances vary from one object to another.

The air gap between the tip of the booster output rod to the back of the MC piston (when the pedal isn't being applied) should be .005"-.010".

Here is a simple procedure for measuring/adjusting the booster output rod length to the MC being installed.

http://redirect.viglink.com/?format=...FMMBAK-1r1.pdf

As was previously mentioned, if you're having to top the MC off, that fluid is leaking out somewhere else. The MC being new doesn't automatically mean it can't be bad right out of the box, so don't count this out as a potential problem. If this is the problem and if it's leaked brake fluid over into the booster, the booster will have to be replaced too. Brake fluid will break down the rubber diaphragm inside the booster and cause it to eventually fail.

The rear drum brake shoes also need to be adjusted out before you'll get a firm pedal and minimal forward pedal travel when applying the brakes --assuming there are no fluid leaks in the system and there's no air in it.

 

After rechecking the rear wheel cylinders I finally noticed a puddle. Earlier in the day I saw it but dismissed it as a water puddle....Silly me. It's brake fluid. That's the good news. Bad news is it's behind the aux tank so that will need to come out. So yes....I have a leak. Feeling rather frustrated.😣Thank you all for the help and advice. It's now beer 30.

 

That's probably the best thing you can do for now. Tackle it again tomorrow. Being frustrated with something usually leads to more mistakes and even more problems that consumes your time and money. Enjoy your adult beverage

 

Thanks. It's more like fall back and regroup..Get a plan for the next round. That and the time to do it. Fortunately the parts won't be too expensive.

 

The good news: it's not your m/c or booster!

 

Yeah I've pretty much ruled all that out. Found the smoking gun so now to get it fixed. My poor wife just doesn't understand how project vehicles are. Hehe.

 

UPDATE UPDATE:
Got the aux tank out and found a ton of dirt and debris on the frame rail along with a nest of some sort that was trapping moisture which corroded the brake line. Got a new section of brake line from NAPA for $7.51. Also, while the tank is out I'm replacing the rubber fuel lines and the filler hoses as well. Ordered those from JBG and are in transit to my door with an ETA of Friday. Weather and work schedule permitting I hope to have it back together on Saturday.