Brake problems

[Randy Bush]

@dgr

Hmmmmmm ok a good manual hydraulic test is in order then

While the truck is off , press repeatedly till all the vacuum is out and you have just a hard/firm pedal and you can’t hear anymore hissing from the booster

STAND on the brake pedal with 134 Nm of force which is about 99 ft/lbs for NO MORE than 3 seconds at a time (repeat 2 or 3 times..... let brake pedal rise fully after each attempt ) . If the pedal falls you have a leak and your hydraulic system is failing if the pedal doesn’t fall then assume it is sound and move onto vacuum testing your system.


Also I think the rod shouldn’t be adjusted outside factory spec , this is when catastrophic damage happens to the MC

doing this pedal does not fall, solid each time.

 

[Slicknik]

@Randy Bush ok great hydraulics test passed for now

you need to do a vac test on your system next

 

[Randy Bush]

@Randy Bush ok great hydraulics test passed for now

you need to do a vac test on your system next

Did check vacuum.# 15

 

[Slicknik]

@Randy Bush can you clarify your response , is that 15 in/hg ?

If it is then I believe you have a leak because that’s very low should be getting 22+ with a new vac pump,

Ok 1. What happens to the needle when you press the brake at idle?

2. After you turn the truck off does your system hold vacuum? If so wait 15 minutes come back and see if the needle has dropped

3. Disconnect the hose that runs to the vacuum pump to isolate it from the system and see how much vacuum your pulling at idle , this should be at around 24 in/hg

 

[dgr]

Randy
Please see if the master cylinder ports are open when the pedal is relaxed. First little press of the pedal, the fluid will flow INTO the m/c until the seals cover the ports.

Do this with the engine off and separately with it running. A bad booster can cause the pedal to not return fully. This would look the same as having too long of a pushrod in there. But would only show with the engine running.

On the off chance is is blocking the ports, pull up on the pedal and then press it when the engine is running.

I think the vacuum reading is low but it isn't clear to me how that would cause a sinking pedal

 

[Randy Bush]

@Randy Bush can you clarify your response , is that 15 in/hg ?

If it is then I believe you have a leak because that’s very low should be getting 22+ with a new vac pump,

Ok 1. What happens to the needle when you press the brake at idle?

2. After you turn the truck off does your system hold vacuum? If so wait 15 minutes come back and see if the needle has dropped

3. Disconnect the hose that runs to the vacuum pump to isolate it from the system and see how much vacuum your pulling at idle , this should be at around 24 in/hg

With the truck running it is 21-22 in/hg , which is fairly normal for here , 3500 ft elevation.

 

[Slicknik]

@Randy Bush

To test for a bad/faulty booster ,(vac gauge hooked up )
when the truck is running , press down on the brake and hold it, see if needle moves , do this test once lightly then as if you are at a light

When you release the pedal and it returns to its resting position pull up or towards you and see if your vacuum level changes

 

[Thewespaul]

Your test for the hydraulic system is incomplete, you need to perform the test with truck running and the vacuum line disconnected from the booster. This will add the rabs valve back into the circuit with the truck running, and allow you to see if it’s affecting the stiffness of the pedal, commonly missed with these trucks.

 

[Randy Bush]

Will have to deal with this more after get back from vacation, thanks for all the thoughts so far.