New brakes issue

[vwaddict]

Hi All, i was wondering if anyone has had the same problem as im having, ive searched the forum 1st & not really found much......ive recently replaced all my hard brake lines & flexis & 1 front caliper. The issue im having is the brake pedal is soft when 1st pressed then after a couple of pumps its solid, then drive it 100 yards its soft again untill pumped or just next time i press the pedal........ This is with the engine running or off, theyve been bled twice & thats including the clutch (which is fine).....
I didn't have this issue when i done the last bus......any idea's

[kevtherev]

I had this and it turned out to be badly adjusted rears.
If you pull the hand brake on does it cure the problem?

Failing that you have to suspect the MC or still air in it.
Can't be much else

[ELVIS]

WHS^

pump up brakes to centre shoes then adjust before you bleed them otherwise all the first one or two pedal strokes is doing is moving shoes out rather than applying force.

[California Dreamin]

.ive recently replaced all my hard brake lines & flexis & 1 front caliper.

Could be rear adjustment, however, ^^^^^^^^^^^ there is no mention of that part being worked on.

Martin

[kevtherev]

.ive recently replaced all my hard brake lines & flexis & 1 front caliper.

Could be rear adjustment, however, ^^^^^^^^^^^ there is no mention of that part being worked on.

Martin

Failing that you have to suspect the MC or still air in it.
Can't be much else

Thanks for your input

[California Dreamin]

Traditional 'mechanical' bleeding of the brakes (by pumping the pedal up and down whilst opening and closing of a bleed nipple) isn't ideal. The 'tandem' seals in the master cylinder travel further than they would normally and ride up and over wear ridges within the cylinder, this can damage the seals themselves. Also, the vigorous movement of the fluid promotes bubbles. The best way to bleed is with a pressure bleeder that gently fills and pressurises the reservoir, all you do then is open each bleed nipple to bleed.

Martin

[Bubble Meister]

How does the vigorous movement promote Bubbles? Surely you want Bubbles - air molecules all together and easy to see during a bleed. After major disturbance air is a pain in the a*** as it will trap at high places or where there is little flow. You may have to bleed multiple times to eradicate it. Also during each bleed there is waste oil which is a cost so there will be a compromise of a very small amount of residual air against the cost of wasting lots of oil to get that tiny amount of air out - that's assuming good standards by not re-using contaminated oil. I've used both bleeding methods on vehicles and aircraft and not always managed to shift the air first time esp after major disturbance. I recently did an independent inspection (fresh eyes and not involved during the actual maintenance) of an aircraft brake system after a component change. The lads bled a considerable amount of oil from the brake pack and still no bubbles. I was getting rather concerned. We bled the other side and loads of air rushed out. I got the lads to operate the brakes 50 times before another bleed on the original brake pack. Finally a small amount if air passed and I was satisfied that the brake system was air free. I was rather relieved as the airworthiness of the system's function rests on my sign off! (Ps you don't get spongy brakes on aircraft with fly by wire brake control - you get the air out and monitor the brake line pressure. If that's ok the next check is when the pilot does a taxi run to the runway.)

[California Dreamin]

How does the vigorous movement promote Bubbles? Surely you want Bubbles - air molecules all together and easy to see during a bleed)

Ahhh! I can see by your name you like bubbles lol...
Vigorous movement i.e. shaking of the brake fluid during transport and during the multiple 'top-ups' each of which pours fluid from a height, the opening and closing of the ports and the seals that cover and under the fluid inlets during manual bleeding. There is always the chance of being caught out by allowing the fluid level to go 'too low' in the reservoir. There are a number of ways to introduce 'aerated fluid'.
When a pressure bleeder is used the risk of introducing air is minimised and those tired seals aren't forced over potential wear ridges in the master cylinder.

Martin