10/4 over and out, shooting for the master cylinder then since i cant find any leaks, and have seen people have to replace them with only 40 k and mines at 182,000

 

ok got the reman master cylinder sittin on my desk (carquest delivers!!!!), starting to have second thoughts about swapping it now. should i check the adjustment of my rear brakes first and go from there, or maybe try another bleed job? do i adjust the pushrod from brake booster to the back of the master cylinder while its apart? help, dirt bike dave or mkosu04 you two have had good suggestions so far?

 

ok got the master cylinder replaced last night, i bought a vacuum bleeder on the way home, not too impressed with it. brakes are improved but i need to bleed them one more time.

 

Glad to hear they are feeling better. Another bleed is a good idea too... a good bleed can make a large difference

 

a guy in the shop told me i should gravity bleed em? says i should crack all the valves till they drip then shut em and bleed em indivually, ever heard of that

 

ok got er fixed up, my rear brakes were way out of adjustment! adjusted em up and bled the lines again, now i have a firm pedal!!!!!!!! so much for self adjusting brakes?

 

Hi folks,

Similar issue, so I thought I would tag on my experiences in here.

1986 F-150, 302 CID, Propane.

Brakes gave up on me after my 3rd load of mulch this summer. Pedal went straight to the floor. Checked Master Cylinder initially and found no Dot 3 left in the larger reservoir.

I have since replaced the Master Cylinder and bled the brake system - the Master Cylinder was leaking fluid out the back and down on the booster.

Master Cylinder has been bled to specifications and the system checks out with the truck off. As soon as I start the motor and vacuum pressure sent into the system - the brake pedal has no resistance and travels to the floor easily.

I am now thinking that the booster is shot - not a big deal to replace, just want to see if any of you learned folks can think of another system that I am forgetting about.

Cheers

 

It sounds like the booster is not maintaining vacuum with the engine off... an issue, but not the key culprit.

You say the pedal feels ok when the engine is off. If your engine is off, and the check valve doesn't maintain vacuum, the pedal would feel real stiff with the engine off. The reason that the pedal goes to the floor with the engine running is that the vacuum in the booster is compounding your brake pedal pressure (generally by about 10x).

My guess is that you still have a bad bleed and there is air in the system. With the engine off, the pedal feels stiff as you cannot compress the air without the assistance of the booster. Once you start the engine, the boosted force on the MC is enough to compress the air in the system and your pedal goes to the floor.

BEFORE bleeding - check all fittings (including bleeder valves) for leaks. Any leaky connections could also contribute to the pedal going to the floor (although you'd probably have found the brake fluid all over by now?).

BTW - I'd also look into getting a new check valve for your booster. It should be rather cheap (I'm thinking $5-10) and will keep the vacuum in your booster even when the engine is off (enough vacuum for several brake applies).

ALSO - I'm assuming you are using a vacuum booster in my comments here... is this correct??? I have no real knowledge of Propane engines so I'm not sure if you have to use a hydraulic booster??? Most of my comments would still apply to a hydraulic booster, except the check valve comments.
(edit - I see you mention vacuum pressure so I guess you do have a vacuum booster)

 

Thanks for the reply "mkosu04".

AFAIK the braking system on my truck is no different than any other stock F-150 for that year.

With the engine off - I can pump the brakes about 4-5 times before the pedal gains stiff resistance and doesn't want to give any more.

I will look into replacing the check valve - could the rod inside the booster need adjusting too? I am wondering if once vacuum is applied to the booster, the rod doesn't travel as far forward as it should to actually touch the piston on the master cylinder. No brake resistance with motor on = pedal to the floor, no braking action at all.

My repair manual doesn't cover general maintenance on the booster - is the check valve easy to remove? Anyone have a good URL covering it?

Thanks again guys - I enjoy working on my truck - it's almost as old as the plane I fly.

 

If that is the case, the check valve might be ok. Is this only immediately? Or does it also maintain vacuum overnight? A good check valve will maintain vacuum for at least 24 hours. No matter how new the part, if you let the vehicle sit for multiple days it will likely loose vacuum

Highly Doubtful - when you apply vacuum to the booster it will actually push the rod farther forward. This is due to the vacuum pulling the front shell back a slight amount and putting a forward force on the internal diaphragm. When we make measurements on test boosters, we measure the output rod depth with vacuum applied (we have special tools that can maintain the vacuum and make a measurement).

Again, my best recommendation is to check for any leaks in the system and then bleed the brakes to make sure there is no air in the system. If that doesn't work, perhaps you got a bad master cylinder when you replaced it???

 

I seem to be having the same sort of problem with my 1998 F-150 5.4L 4x4. I have been having a really spongy brake problem. I have Replaced front rotors, pads, master cyclinder, Bleed Brakes Multiple times, no loss of fluid anywhere, rear shoes are fine. Could it possibly be a bad brake hose? I believe the brake hoses are double walled, if the inner wall uptures the fluid could be buldging the outerline usually the rubber coating? Any ideas or experiences with this sort of thing?


Tom...