Xoutpost.com - Brake booster going or vacuum pump?

Quote:

Originally Posted by crwarren11 (Post 1083556)

No. Air was not the problem initially. Everything felt completely different after the new booster. Air became a secondary problem that I could tell was an issue afterwards. The brake performance and power was excellent after the new booster. There was just no resistance felt in the peddle for the first inch. Which is what I consider a typical air in the system symptom.

Quote:

Originally Posted by Junkycosmos (Post 1083575)

For those trying to sort out booster vs other issues. I would remind that the X brakes have two separate systems at work here. The hydraulic system: which is the master cylinder, lines out to brakes etc. The other is the booster side: vacuum from the engine (intake or vacuum pump, 1 way valve, and booster (big diaphragm). The hydraulic side should "feel" solid pedal, not spongy when pushed. You can test the hydraulic side with the motor off as after 2-3 pedal pushes the vacuum booster will be out of residual boost. With the motor on you can test the booster: all it does is help you depress the pedal.

So:
1. soft or spongy pedal = issue on the hydraulic side (probably air in the brake lines). X system is hard to bleed after master cyclinder change: for me it took running the procedure on GT1 that activates the abs pump twice.

2. pedal firm but very hard to press: like it takes 2 feet to stomp on pedal even while engine is running. Maybe revving engine in neutral helps pedal be better. This is a booster issue. Common on the V8 BMWs: few tsbs by BMWs and limited recall on 4.8 motor but did not include many X5s. If your find oil in your vacuum line past the 1way valve then it's likely in your booster and the booster will fail.

Hope this helps and best of luck.

J

This makes sense but is the opposite what crwarren11 describes, i.e. his brake was not hard, rather, for an inch no resistance, typical of air in the lines yet changing the booster is what he said fixed his issue;

The main reason I don't believe I have air, despite the increased pedal travel, is because I intially never opened the hydraulic system so I don't see how air could have gotten in-
I took pads out ,
pushed caliper pistons back in to accommodate the new pads,
then - low pedal.
I hoped it would fix itself after just driving for some days,
no change,
then I tried bleeding - no change;
I thought maybe I've damaged the master cylinder by pushing the pistons back in the calipers, so swapped in a brand new BMW master cylinder- pre-bench bled, then completley pressure bled the entire system using 2 gallons of brake fluid, whilst activating the ABS via the laptop-
no change-
going back to where I started, it's hard to describe, but it doesn't 'feel' like air and how could it have been initally?
Pressing the brake pedal, I'm definitily getting power assistance from the booster, but I can hear a faint hiss as the pedal drops,
but like crwarren11 , perhaps just changing the booster will fix the issue, but I don't feel like doing it just for fun, like I did with the MC- that was a let down when the brake pedal was the same after all that...:bawling::banghead: