Buzzing under hood - brake pressure unit?

[RL4x4]

The round cylindrical brake pressure unit under the hood, near the firewall on my '01 non-AHC rig makes a fair amount of buzzing noise.
I notice it most when I first start the vehicle and then sometimes at lower speeds when applying the brakes.

What function does this unit perform?
Is it normal for it to buzz/vibrate at random intervals?

 

[OregonLC]

That is the pressure accumulator for the brake system. A pump periodically runs to build pressure. The accumulator 'stores' this pressure until needed by the system. The buzzing you hear is the power brake pump. I sometimes hear mine at startup but then never again, although I'm sure it runs throughout the drive. how often? I have no idea.

 

[NMuzj100]

At startup and random intervals - Normal.

At every brake application - Not Normal.

Do your brakes work well?

 

[OregonLC]

So... how often is 'normal'? I was playing around today and find that key on, vehicle not running I can get 4 good hard pumps before it kicks on. It runs for 5 seconds or so then turns off. Completely repeatable each time. FSM doesn't mention how many pumps is good, but it does talk about needing 40 pumps as part of the bleed cycle. Anyone care to try the same and report back?

Wondering if I have air trapped in the system? Hopefully not a bad booster pump.

Anyone ever use a reverse bleeder on their LC/LX? I've heard great things about the phoenix. Phoenix Systems | V-12™ The Economy Professional Model Brake Bleeder

 

[OregonLC]

I'm going to answer my own post: it appears that 4 pumps between pump cycles is normal. my fully scientific survey of two cruisers proves this . actually, on my buddy's 03 it was about 3 pumps between pump cycles.

After reading about the 40 pumps in the FSM bleed procedure I was thinking that there should be more reserve in the accumulator than just 4 pumps.

So with a faulty boost system ruded out, it's on to braided lines and pads.

 

[RL4x4]

I'm going to answer my own post: it appears that 4 pumps between pump cycles is normal. my fully scientific survey of two cruisers proves this . actually, on my buddy's 03 it was about 3 pumps between pump cycles.

After reading about the 40 pumps in the FSM bleed procedure I was thinking that there should be more reserve in the accumulator than just 4 pumps.

I like your test idea!

With the engine off, ignition on, I get 3 pumps of the pedal before the pump cycles on. I repeated the test four times and got the same result each time. Does this indicate that one of the brake system components is faulty or that there is air in the system? What is the "40 pump" bleed procedure?

FWIW, I find the brakes work very, very well for such a heavy vehicle and the brake pedal feel is firm and consistent.

 

[RL4x4]

Bump for more info.

Has anyone else performed the stationary pump test?

 

[uHu]

3-4 times fully applied brakes sounds normal. The number of times will depend on how much the pads retract from the disk each time.
Even though the booster pump starts again after 3 or 4 times of braking fully, there is still enough pressure to brake fully several more times, like 20 times total.

The 30 to 40 "steps" is for emptying all pressure completely. (and btw: that's when you check the brake fluid level)

Another issue is how long the accumulator will store the pressure. Like: Does the booster pump start when you turn the ign on after standing still for one hour? (without touching the brakes). What about over-night? For how long will it run then?