Brakes- loss of pedal, no apparent leaks.

[Orkea6]

Hey Guys,

First post here, grateful to Gary for building this site.

My 84 F150 with 4.9L and 4 speed OD had a sudden loss of brake pedal- it sure felt like the many times I've had a master cylinder fail over the years- but it seems like that is not the issue. I check the truck first thing for any signs of leaks at each wheel. None noted. I then took my brake line clamps and first closed off the front calipers- no change (still poor pedal). Then I added the rear calipers to the clamping (clamped the line going to the wheel cylinders) - no change. So it seemed the culprit might be the MC. To be sure, I installed the plastic caps which came with my new MC (replaced about 5 weeks ago) and expected a soft pedal- but instead I have a very firm pedal.

What is recently replaced:

Left and Right Calipers

Front Left and right brake lines

Rear Center brake line

Rear Left and Right metal lines

Left and Right rear wheel cylinders

Rear Shoes

All rear hardware

Left and Right Parking brake cables

Park brake return spring

Master Cylinder

The above were replaced as PM as the original MC was rusty and not confidence inspiring (among many other things). I bled the brakes and have put about 500 miles on them, not super good pedal but seemed fine.

I bought a tool for adjusting the power booster rod depth and adjusted that to spec when I installed the new MC- but the pedal was never really as firm as new.

Original Parts:

Front metal lines

Prop/Warning Valve

Power Booster

My tests for Power Booster were:

Engine off, pump pedal until stiff.

Start engine- pedal should drop (passed)

Turn engine off with pedal depressed- doesn't sink after 45 seconds (check valve OK).

It would seem unlikely the metal lines are at fault, also the prop valve if failed would divert to front or rear only- and the brake light should be on (it is not on). So at this point it seems the only rubber suspect is the power booster- can a bad booster give a soft or no pedal? I guess if the booster bladder is ballooning and not leaking, it can. I once had a E-350 which I could not get a firm pedal. No matter how much I bled it- the pedal was always soft. I thought it might be the rod adjustment, so I took it to my mechanic and it ended up being the booster ballooning inside the housing... but this still to me seems not likely. Do any of you guys know how to test it for sure- not just vacuum leak- but all failure modes? I would like to know for certain, rather than just replacing parts and hoping I guessed right.

Sorry for such a long post- thanks for taking the time to read and thanks further for any responses.

Bill

 

[mat in tn]

in this type of scenario i always. look at bleeding the system. common diagnostics would say "look at the last item touched" typically speaking the soft pedal is due to a lack of hydraulic resistance not a greater one.

if you had any cylinders or calipers stuck, they would still give resistance even if they did not transfer the pressure to the pads or shoes. if it were a proportioning valve it would just move pressure from front to rear or vice versa. leaks would relieve pressure, but you should see them easily. and you said that you have looked for that. brake booster makes braking easier and if it fails you will have a stiffer pedal and often a hissing as it usually results in a bad vacuum leak. it's time to look at what you cannot see.

[85lebaront2]

in this type of scenario i always. look at bleeding the system. common diagnostics would say "look at the last item touched" typically speaking the soft pedal is due to a lack of hydraulic resistance not a greater one.

if you had any cylinders or calipers stuck, they would still give resistance even if they did not transfer the pressure to the pads or shoes. if it were a proportioning valve it would just move pressure from front to rear or vice versa. leaks would relieve pressure, but you should see them easily. and you said that you have looked for that. brake booster makes braking easier and if it fails you will have a stiffer pedal and often a hissing as it usually results in a bad vacuum leak. it's time to look at what you cannot see.

On thing I found the hard way on Darth, didn't hit anything and it fortunately happened after I had parked the 5th wheel at a campground. I went to go somewhere and the pedal went down slowly and finally stopped with the brake light on. The brake lines on these trucks run down the inside of the rear left side rail, going up over the axle kickup, then back down into the corner where the crossmember behind the axle joins the side rail where it makes a 90° bend. This bend is located in a pocket where dirt and water accumulate. Try to find the correct combination of lines and fittings in the boonies on a Sunday.

[FuzzFace2]

in this type of scenario i always. look at bleeding the system. common diagnostics would say "look at the last item touched" typically speaking the soft pedal is due to a lack of hydraulic resistance not a greater one.

if you had any cylinders or calipers stuck, they would still give resistance even if they did not transfer the pressure to the pads or shoes. if it were a proportioning valve it would just move pressure from front to rear or vice versa. leaks would relieve pressure, but you should see them easily. and you said that you have looked for that. brake booster makes braking easier and if it fails you will have a stiffer pedal and often a hissing as it usually results in a bad vacuum leak. it's time to look at what you cannot see.

First you did a lot of work to re-new the system good for you as a lot dont

Did you adjust the rear brake shoes?

If not that will show as a low pedal.

If you did adjust them and then drove it for a bit try adjusting them again as any high spots would wear down adding play.

If that dose not pan out how did you bleed the system?

Use 2 people and started at the far wheel, RR, LR, RF, LF.

Now if that is how you did it try again.

Now if that dose not work you will need to pressure bleed it as that is the only way you are going to get the air out.

It was the only way I could get a pedal on my truck and my system was open for years and everything replaced like yours.

Dave ----

[Orkea6]

in this type of scenario i always. look at bleeding the system. common diagnostics would say "look at the last item touched" typically speaking the soft pedal is due to a lack of hydraulic resistance not a greater one.

if you had any cylinders or calipers stuck, they would still give resistance even if they did not transfer the pressure to the pads or shoes. if it were a proportioning valve it would just move pressure from front to rear or vice versa. leaks would relieve pressure, but you should see them easily. and you said that you have looked for that. brake booster makes braking easier and if it fails you will have a stiffer pedal and often a hissing as it usually results in a bad vacuum leak. it's time to look at what you cannot see.

for bleeding I used both the vacuum bleeder system, and followed with the old fashioned two person pump and bleed- starting at the closest to MC (LF) and ending at the far right wheel. Thing is- the pedal pumps up with the truck not running. Start the truck, and it disappears.

I do have a Motive pressure bleeder and I need to get a adapter plate for this type MC. An air pocket doesn't make sense as they don't just wait 5 weeks to suddenly show up.

Thanks for the help.

[Orkea6]

On thing I found the hard way on Darth, didn't hit anything and it fortunately happened after I had parked the 5th wheel at a campground. I went to go somewhere and the pedal went down slowly and finally stopped with the brake light on. The brake lines on these trucks run down the inside of the rear left side rail, going up over the axle kickup, then back down into the corner where the crossmember behind the axle joins the side rail where it makes a 90° bend. This bend is located in a pocket where dirt and water accumulate. Try to find the correct combination of lines and fittings in the boonies on a Sunday.

I've inspected all the metal lines- again; no loss of fluid. no apparent damage externally. Thanks for the suggestion

[Orkea6]

First you did a lot of work to re-new the system good for you as a lot dont

Did you adjust the rear brake shoes?

If not that will show as a low pedal.

If you did adjust them and then drove it for a bit try adjusting them again as any high spots would wear down adding play.

If that dose not pan out how did you bleed the system?

Use 2 people and started at the far wheel, RR, LR, RF, LF.

Now if that is how you did it try again.

Now if that dose not work you will need to pressure bleed it as that is the only way you are going to get the air out.

It was the only way I could get a pedal on my truck and my system was open for years and everything replaced like yours.

Dave ----

Pedal builds pressure with truck not running- not with it running. I think I may have just answered my Q right there.

Bleeding started at closest -(LF) to the furthest (RR).

Shoes were seated, adjusted by rotating the wheel until it stopped, then backed off 20 clicks. Then checked again after first day test drive.

[FuzzFace2]

for bleeding I used both the vacuum bleeder system, and followed with the old fashioned two person pump and bleed- starting at the closest to MC (LF) and ending at the far right wheel. Thing is- the pedal pumps up with the truck not running. Start the truck, and it disappears.

I do have a Motive pressure bleeder and I need to get a adapter plate for this type MC. An air pocket doesn't make sense as they don't just wait 5 weeks to suddenly show up.

Thanks for the help.

The vacuum bleeding if from a hand pump is a wast of time in my book.

I have one that uses shop air to make vacuum and that was only a little better.

When bleeding ALWAYS start at the farthest wheel and working to the master.

Check the rear brakes to make sure they are adjusted if not that can through off the whole system.

Get the needed MC cap and pressure bleed the system.

If the rear is adjusted and the pressure bleeding did not help I am thinking the new MC is bad.

If you can fid an empty lot get up to speed and lock out the brakes and see what marks are left.

If all is good you should have 4 even marks.

If only 2 wheels lock, front or rear, then you know where the system has the issue.

Dave ----

[mat in tn]

for bleeding I used both the vacuum bleeder system, and followed with the old fashioned two person pump and bleed- starting at the closest to MC (LF) and ending at the far right wheel. Thing is- the pedal pumps up with the truck not running. Start the truck, and it disappears.

I do have a Motive pressure bleeder and I need to get a adapter plate for this type MC. An air pocket doesn't make sense as they don't just wait 5 weeks to suddenly show up.

Thanks for the help.

The vacuum bleeding if from a hand pump is a wast of time in my book.

I have one that uses shop air to make vacuum and that was only a little better.

When bleeding ALWAYS start at the farthest wheel and working to the master.

Check the rear brakes to make sure they are adjusted if not that can through off the whole system.

Get the needed MC cap and pressure bleed the system.

If the rear is adjusted and the pressure bleeding did not help I am thinking the new MC is bad.

If you can fid an empty lot get up to speed and lock out the brakes and see what marks are left.

If all is good you should have 4 even marks.

If only 2 wheels lock, front or rear, then you know where the system has the issue.

Dave ----

this certainly seems to me to be either air in the lines or a failing in the master cyl.

[Lima Delta]

Pedal builds pressure with truck not running- not with it running. I think I may have just answered my Q right there.

Bleeding started at closest -(LF) to the furthest (RR).

Shoes were seated, adjusted by rotating the wheel until it stopped, then backed off 20 clicks. Then checked again after first day test drive.

Coincidentally I started getting the same symptoms over the weekend.

Still not sure what's going on, but I'm thinking it's the master cylinder.

Brakes are adjusted, and I haven't done any brake line work since last year so I have no reason to suspect air in the system...

However when I was going through the diagnostic procedure in my shop manual, I noticed in the rear brake reservoir:

- If I press the pedal slowly, I am seeing the fluid level rise slightly then go back down when I let go which indicates a problem with the master.

- If I press and release the pedal more quickly I am getting a geyser which indicates air in the line.

Is it possible the master is letting air in?