Brake bleeding

[timmy201]

I had 30 minutes this morning and decided to have a look at the caliper as per Graeme’s suggestion.

Started by pulling out the brake pads one by one (which were still loose and rattled when I pushed them) and then pressing the pedal down so that the pistons would work their way out. I lubricated the outside of the piston with some brake fluid and worked them back in again. I didn’t push the pistons all the way in this time, just enough so the pads were a nice tight fit. This has found a new resting position for the pads and now the pedal is nice and firm from the top!

I wasn’t too worried about any air bubbles in the master or rear brakes as these haven’t been touched recently. I’ll try bleeding the front calipers when can get someone to give me a hand (foot)

Lessons for the day:
Pistons retract using the flex in the seal and need to be worked in and out to find their appropriate resting position that allows the pads to touch the disc straight away
Seals need to be worked back and forth with some brake fluid (not just the brake assembly grease) to get the pistons sliding nicely

Thanks all for the help. I’ll do the same process to the other caliper, just to make sure it’s all happy. On the road soon!

[Peter Laidler]

Head above the parapet time..... According to my very limited knowledge of all things mechanical and hydraulic, I say that NO brake fluid should EVER pass the square inner seal whether by accident or design. This is the fluid seal. The two-lipped OUTER seal, held in with the retaining ring (for a good reason too…..) should remain totally dry - as should the caliper pistons from the square seal outwards.

No need to read further if you already know all this. When you press the pedal, the fluid forces the 4 pistons outwards, equally.
Simultaneously to this happening, the two little lips of the outer two-lipped seal flex slightly outwards with the piston, towards the pads which are slowing down the car. So far, so good.

But when you release the pedal and therefore pressure, you’ve got to ensure that the pads release their pressure – or lateral loading - on the discs and that the pistons release their pressure – or lateral loading – on the pads. This happens due to the fact that the two lips, being flexed outwards slightly now reassert themselves, to resume their natural position and by being in tight contact with the piston, withdraw the piston a gnats knacker (or RCH for the Antipodeans out there). So, piston now withdrawn = brake pad free of the disc!

It goes without saying that if your outer seals are greased or oiled, the two lips won’t fully grip the piston…., no, they’ll just slide along it. They won’t be pulling the piston back from contact with the disc.

Some just don’t get this, even as apprentices or students it was difficult to get our heads around it. But the same practice applies to BIG, and I mean BIG artillery recoil and recuperation systems. You can easily see the double lipped seal at work (we used to call them 'tipper' seals). Take caliper off car but still connected to system. Insert a piece of wood to replicate a brake disc and look carefully while you press the brake pedal. Pistons move outwards. Release pressure and then see the pistons being withdrawn slightly. This ain’t no white mans magic. It’s the two-lipped seals doin’ their job by friction! And if they’re oiled or greased, they won’t withdraw as they should and your pads are wearin’ out faster.

[greyghost]

good to see it is an easy fix

[andy1071]

Sorry Peter, I have to correct you.. Right principle, wrong seal..

The outer double lip seal is 'just' a dust seal.
The retracting force on the pistons is due to the main pressure seal. This seal is square-section shaped, but the seal groove is tapered, being slightly smaller on the pad side, and bigger on the hydraulic side.
When you press the pedal, the seal is deformed in the groove towards the pad, and when the pedal is released, the seal moves/expands back slightly in the groove. It is the main seal that pulls the piston back, as you say, by a knats nadger.

[MiNiKiN]

I second Andy's statement. That is exactly what I was taught in my apprenticeship and master craftsman trade school

[Peter Laidler]

PM's sent to both with my quals for saying this.

[andy1071]

Peter, I haven't received a PM.
But I do feel I am rather qualified for stating the function of the seal, as I am an ex-AP employee (AP = Automotive Products = Lockheed & Borg and Beck).
I did my apprenticeship there (what feels like 100 years ago... ), and subsequently worked in AP Racing designing brakes and clutches.

[MiNiKiN]

From my textbooks with translated text

Brake_Calliper_function .jpg

[andy1071]

That textbook is wrong..!
-If the seal is extruded between the piston and bore as shown it will cut off small chunks 'with each actuation' and damage the seal (this can, and does, happen if the clearances are too high, and if the seals are too soft -for example, due to age/contamination).

The 'taper' of the seal-groove is quite subtle, and took some time to develop.
-AP also had their own Rubber Division and developed all their own seal compounds, so everything worked together.

Peter, I fully agree with you that an ANY seal can be used to draw back a piston.
But in this specific brake application, it is, as I say, the main seal that does the job. The outer seal cannot (in the Lockheed brake caliper) exert enough force to draw the piston.
Maybe other Brake manufacturers use the outer seal for that function, but AP/Lockheed used the main-seal.