Brake pedal feel different on first brake application

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Re: Brake pedal feel different on first brake application

Post by AndriesS » 14 Nov 2018 08:33

Differentiate between softer and longer travel? i.e. when you reach the firm pedal point on first attempt is it the same firmness as on the 2nd application

1) Does it happen only while you are driving
2) Does it happen while not moving with engine on?
3) Does it happen with engine off - and not moving? - Do press the brake a couple of time after switching off

from bottom
3) eliminated the vacuum booster , if travel changes from the first to the second the problem is most likely hydraulic, could be air in the system, leaking master you will generally be able to depress the pedal more and more by keeping pressure on the pedal
2) could be vacuum booster, first use adds the power but vacuum leaks or insufficient on second and feels like pedal travel less but actually teh difference is teh power assistance
1) could be that a disk is not running true, or caliper loose and while driving the caliper pistons are moved further away, thus on first brake application you need to take up the movement and have more pedal travel, however quick subsequent brake application the piston have not moved as much back and the travel is less

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Re: Brake pedal feel different on first brake application

Post by mvcoller » 14 Nov 2018 10:47

Start bleeding at the furtherest point from the reservoir and work your way to the closest. That is the correct order. Much easier way to remember the bleeding order!

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Re: Brake pedal feel different on first brake application

Post by Florian320 » 14 Nov 2018 10:54

Hi Andriess,

I‘ll try to work it out according to your hints.
What I can say already is that the reservoir is just a tad above max, so there shouldn‘t be any leak.

Thanks for the update regarding the bleed: I‘ll pass that through to my workshop.

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Re: Brake pedal feel different on first brake application

Post by iandvl » 14 Nov 2018 11:10

mvcoller wrote:
14 Nov 2018 10:47
Start bleeding at the furtherest point from the reservoir and work your way to the closest. That is the correct order. Much easier way to remember the bleeding order!
I checked the manual, and it is as I outlined in my earlier post.

1: Load sensing valve (LSV).
2: Back left.
3: Back right.
4: Front left.
5: Front right.

The LSV is not the furthest point from the reservoir.
bleeding.png

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Re: Brake pedal feel different on first brake application

Post by Florian320 » 14 Nov 2018 12:06

Thanks!

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Re: Brake pedal feel different on first brake application

Post by AndriesS » 14 Nov 2018 12:50

I'm always hesitant to bleed brakes on older cars, even more so if you don't know the history. The problem is that when bleeding you all of a sudden use a part of the master cylinder that would not have seen any use it a number of years and any deterioration of that surface could potentially damage your master cylinders seals causing problems.

This happened on a 20 year old car that might not have had it the brakes bled or fluid exchanged in 15 years if not more. when I opened the m/c you could see a shiny part , then dull , then shiny then dull( dual m/c) and i believe that when the bleeding was done the seals moved over the unused part causing damage.

Thus I prefer to bleed using vacuum at the caliper or pressurizing the system from the reservoir side and if manual bleeding is required to put a spacer / block under the brake pedal as not to push it to the floor.

I'll gladly hear your opinions on it.

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Re: Brake pedal feel different on first brake application

Post by SCUBA Patrol » 14 Nov 2018 13:07

Makes sense Andries, seeing that hidrolic fluid is hygroscpoic, it absorbes water, and thus the rust effect on break parts.
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Re: Brake pedal feel different on first brake application

Post by Florian320 » 14 Nov 2018 15:06

That‘s an interesting point!
I think -but have to ask to be sure- that the car got fredh brake fluid 1 or 2 years ago.
I just tested after driving with engine off: on first press I could depress the pedal with little force to lets say 30% of travel.
The second pressing was much firmer and I had much less travel, the 3rd try even gave a tad less way I think.

Maybe I should have the discs checked for alignment?

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Re: Brake pedal feel different on first brake application

Post by AndriesS » 14 Nov 2018 21:11

On your first press the booster still had vacuum and you typical feel that you required less force and the travel was a bit more. Your second and third should be the same.

I suggest before you start it tomorrow morning you check the difference in travel between your first three presses,( do them 2 seconds apart) there should be no noticeable difference, if the pedal comes up - I.e 2, 3 and 4 is now firmer/ higher then you could have air in the system.

After this keep your foot on the brake and start the car, you should feel the brake pedal being pulled in by the brake booster, sign that booster is working but not indication if it's leaking vacuum or not

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Re: Brake pedal feel different on first brake application

Post by Peter Connan » 15 Nov 2018 05:08

AndriesS wrote:
14 Nov 2018 21:11
After this keep your foot on the brake and start the car, you should feel the brake pedal being pulled in by the brake booster, sign that booster is working but not indication if it's leaking vacuum or not
But not too hard a pressure. I have burst two brake lines by doing this!
Mag ons ons kenniskry met lekkerkry aanhoukry.

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