engine stumble when brakes applied

[kicker22004]

Also as a side note the truck runs fine without pressing on the brakes. It only acts up when I press down or release the brakes. If the truck is warmed up the idle is lower down closer to 830 and pumping the brakes can cause it to almost die.

[Thomas Perkins]

Check your power brake hose.

 

[kicker22004]

This is my plan once I get a brake in the weather. I've been in the infamous Nissan forums for this truck but figured more minds on similar trucks can't hurt. 

[kicker22004]

I've checked the hose and didn't find a leak. I bought some starting fluid today so tomorrow I can look for leaks easier.

[datzenmike]

Block off the power brake booster hose right at the intake and try. This eliminates the booster as the cause also. Beware, the effort to brake will be much increased.

 

Just for the hell of it what happens when you...

 

Pump the brakes to exhaust any remaining vacuum stored.

Step on brake and hold.

Start engine and observe...

Pedal drops slightly as engine vacuum rises.... this is normal operation, is this what happens???

 

 

 

[kicker22004]

1 hour ago, datzenmike said:

Block off the power brake booster hose right at the intake and try. This eliminates the booster as the cause also. Beware, the effort to brake will be much increased.

 

Just for the hell of it what happens when you...

 

Pump the brakes to exhaust any remaining vacuum stored.

Step on brake and hold.

Start engine and observe...

Pedal drops slightly as engine vacuum rises.... this is normal operation, is this what happens???

 

 

 

With the hose blocked off everything works as expected with zero stumble but stiff pedal because the booster isn't assisting. 

 

With the booster connected and the brakes held before starting the pedal drops once the truck is started.

[Stoffregen Motorsports]

Could it be that the booster is filled with brake fluid and somehow not leaking, but when the brakes are applied, the engine sucks brake fluid?

 

Does the  hose going from the motor to the booster look wet on the inside?

[datzenmike]

2 hours ago, kicker22004 said:

With the hose blocked off everything works as expected with zero stumble but stiff pedal because the booster isn't assisting. 

 

With the booster connected and the brakes held before starting the pedal drops once the truck is started.

 

Well something isn't right there. Replacing the two lengths of hose is the cheapest thing to try first.

[kicker22004]

1 hour ago, Stoffregen Motorsports said:

Could it be that the booster is filled with brake fluid and somehow not leaking, but when the brakes are applied, the engine sucks brake fluid?

 

Does the  hose going from the motor to the booster look wet on the inside?

Good idea, I just checked and no it's dry. I'm heading to town in the truck here soon now that I got shocks on it and blead the brakes. I'm going to pick up some hose and a few clamps.

[datzenmike]

Has to be fairly thick stiff hose, heater hose just won't do, or it will collapse from the suction.

[kicker22004]

7 hours ago, datzenmike said:

Has to be fairly thick stiff hose, heater hose just won't do, or it will collapse from the suction.

Yeah, well after driving it today I noted that it doesn't seem to happen while driving only when parked. I blead the brakes and now everything feels normal so I'm going to move on to getting it smogged again. If I ever figure this out I'll update it here tho.

[datzenmike]

If there is a vacuum leak it won't be noticeable as much when driving. At idle anything that takes away will.