Technical Brake fluid reservoir/master cylinder removal

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Technical Brake fluid reservoir/master cylinder removal

hurjakuru

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Joined
May 28, 2021
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7
Howdy folks!

my B had very low braking power on rear brakes during its yearly checkup(0,7Kn). They somehow passed but i'd still rather have properly working brakes 😀. Since the braking force is equally bad on both sides and i have a repair kit for the master cylinder at hand anyway i thought i'd remove it and change all the seals etc at least to rule it out if nothing else.

So i went ahead with the "how hard can it be?" attitude and quickly ran to a problem of how do i remove the brake fluid reservoir without damaging it? Feels like if i pull any harder its just gonna break.

Is there a correct way of doing this? maybe pry it loose from the rubber seals themselves or something? I can't even find a possible replacement part online so i really don't want to damage the reservoir.

Any additional tips for completing this job are very welcome aswell!
 
Joined
Jan 26, 2022
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Location
Råå
Howdy folks!

my B had very low braking power on rear brakes during its yearly checkup(0,7Kn). They somehow passed but i'd still rather have properly working brakes 😀. Since the braking force is equally bad on both sides and i have a repair kit for the master cylinder at hand anyway i thought i'd remove it and change all the seals etc at least to rule it out if nothing else.

So i went ahead with the "how hard can it be?" attitude and quickly ran to a problem of how do i remove the brake fluid reservoir without damaging it? Feels like if i pull any harder its just gonna break.

Is there a correct way of doing this? maybe pry it loose from the rubber seals themselves or something? I can't even find a possible replacement part online so i really don't want to damage the reservoir.

Any additional tips for completing this job are very welcome aswell!
Best wishes for a successful job! Have you found your way to the b service manual and parts catalog here on this site under downloads (I think)? Comes in very handy, much to often!
 

wapper

Active member
Joined
Sep 30, 2015
Messages
112
Are you sure the master cylinder is the one to blame?
We have the usual diagonal split system, which means rear wheels are on different lines of master cylinder, and should not fail at the same time.

I have similar problem and I have replaced rear calipers, rear brake force regulators, and recently also rear brake hoses. Still need to go and have them check the braking power, to see if something is better now.

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Joined
Nov 28, 2019
Messages
28
I had a similar problem with weak braking power. I replaced nearly everything including the ABS module. It turned out to be the brake power regulator (a faulty new replacement). It might be an idea to get a brake pressure testing kit which you can use to test the pressure in different sections of the braking system. I wish I'd done this at the start and saved a lot of time and hassle.
 
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hurjakuru

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May 28, 2021
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Thanks for the replys!

First an update on the job itself: the brake pipe fittings are stuck. I cant get 3 of them loose without rounding the nuts, already had the wrench slip once. Have to visit the tool shop for this one. Couldn't figure out a safe way to remove the reservoir either so i just put new fluid in and bled all the brakes in hope that they would at least improve.

As expected though there was no noticeable difference.

As far as i know my B does not have ABS, its a 1996. atleast i dont see any of the ABS components in my car. I have 4 brake pipes coming form the master cylinder and you bleed the brakes manually with an air screw.

The reasons i suspect the master cylinder are:
Its only affecting the rear brakes and both of them equally. Front brakes are both 2,2Kn and can be locked if you stomp on the pedal hard enough.
Brake fluid flowed well during bleeding so i don't suspect anything to be clogged.
The handbrake is also equally weak at 0,7kn and i am pretty sure it operates the rear brakes.

So iam thinking the seals inside the master cylinder are leaking pressure and thats causing the front brakes to still function but theres not enough pressure left for the rear brakes.
 

wapper

Active member
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Sep 30, 2015
Messages
112
You could try heating those pipe fittings with a flame tool, with all precautions of course.
The handbrake has nothing to do with master cylinder, or any other hydraulics.

I went to garage and checked my rear brakes by lifting one side of the car, pushing brake pedal with wooden stick and trying to turn the rear wheel by hand. I can confirm that after changing rear brake hoses, the brakes are definitely better now. Before the change I could turn the rear wheel while my wife was pressing brake pedal as hard as she could. Now the rear wheel will not turn after gentle push on the stick.
I guess the old hoses degraded and were either blocking the pressure, or simply expanding too much.

My barchetta is 1996 too, but with ABS.
 
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hurjakuru

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May 28, 2021
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The handbrake has nothing to do with master cylinder, or any other hydraulics.
Interesting! I assumed since the handbrake cabbles are connected to the rear brake calipers. Then there must be a separate mechanism inside the calipers for the handbrake to push the piston? In that case the fault might lie inside the calipers.
 

wapper

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Sep 30, 2015
Messages
112
Yes. The piston is moved by special shaft inside the caliper. You can drain all your brake fluid (or lose it after an accident) and your handbrake will still work.
 
Joined
Dec 5, 2006
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Last year I helped a friend with his Rover 800 where he could barely get any fluid out of the rear cylinders. He had ABS. First port of call was to check fluid came out of each of the ABS ports. Traced the brake lines looking for obvious kinks etc. All OK. In the end it could only be the rear flexi hoses. It was, they were basically totally stuffed internally and not allowing fluid to pass.

So brake hoses can/do degrade. Not sure why backs before front other that more pipework thus more fluid = more moisture = more corrosion?

On a car with no ABS the there will be a rear regulator valve with a link between the chassis mounted valve body and the rear axle. This valve is "in-line" before the "T" split to left and right rear brakes. It's purpose is to *reduce* rear fluid pressure from the master cylinder. The rears always run with less pressure for a normally loaded and level car . Then as the rear axle load increase the regulator increases the pressure. If the rear axle load decreases (eg. under braking) then the pressure is increased. The is normally and adjuster on the actuation lever to set the nominal pressure. Now I think we all know this but the ramifications don't always hit you.

1) If rear hoses are degrading then they are already running at lower pressure so a small degradation in the hose has a much amplified effect

2) Even with good hoses when you find difficulty in getting good fluid flow when attempting to bleed the brakes then if you have jacked the car under the chassis and the axle is hanging down on either of both sides then jack the car up under the axle/hubs so the rear suspension is compressed. This will open the regulator valve up and increase flow/pressure to the rear cylinders.
 
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hurjakuru

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May 28, 2021
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I currently have both rear brake hoses waiting for pickup and i have one of the rear calipers dismantled on my kitchen table right now. There was a lot of gunk inside the caliper housing after removing the piston.
The piston has some minor scratches on it, mainly on the top portion. you can see light reflect from them but cant feel them with a finger, should i consider replacing it/them too? Seems to me that its condition is not that bad, but i have been wrong before.

After blowing the housing clean with an air compressor and repeatedly activating the handbrake mechanism it atleast feels like there is more movement in it. maybe ~1cm more at the tip of the lever.

The seals look good too, so good in fact that i am debating whether i should buy new ones at all. Honestly i am tempted to just put the whole thing back together now that its cleaned and see if there is any difference. Especially since it was only the handbrake that did not pass.

I'll look into the load proportioning valves next but i really hope its not them, they seem to be way more expensive than the pressure regulators meant for ABS versions.

I will continue tomorrow/thursday.
 

wapper

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Sep 30, 2015
Messages
112
Same here, the handbrake cable was one of the first things that needed replacement after buying this car.
What a nice surprise it was, after 10min outside in winter time I wasn't able to pull back to garage, because water and rust inside the handbrake cable were suddenly frozen solid )

Just in case, there is total of 4 rear hoses, two on each side.
 
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It is not unusual to have minor scratches and scuff on pistons. It is the bore that is important. A good seal will also tolerate these on the bore if they are very fine. If if does not then you will have fluid expelling from the calliper.
 
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hurjakuru

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May 28, 2021
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okay i got the first caliper cleaned up and back together. i'll put it on tomorrow and test if it helped. Wasn't able to fully dismantle it as there is a locking ring deep in the housing that seem to require some special pliers to reach. Couldn't find anything that would fit. Hopefully the cleaning+ new piston seals were enough.

Gave the piston a light sanding with p2500 wet sandpaper which cleared out most of the markings.

The cables are definitely intact and in working order. the handbrake does engage, just not with enough force. At this point i am convinced that the calipers are at fault for the weak handbrake. The cable has been tightened and move freely. The pads are almost brand new and the disc is not too bad either so that leaves the caliper.
 
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