Bleeding brakes

[gbb483]

Having rebuilt my rear brakes I am about to bleed them. So it is dual circuit, one for the front and one for the rear (and one chamber of the MC reservoir is currently empty).

So how can I pump the brake pedal to bleed the rear brakes when pressure in the front brake circuit will block the pedal from moving? I also want to avoid upsetting the PDWA.

[trunt]

The empty half is the rear brakes. Bleeding rears is not affected by the fronts still being bled, the pedal will just go down and you will feel resistance when the rear piston reaches the piston for the front. It also works the other way round, that’s how it deals with it should one circuit fail.

Afterwards you will need to reset the piston in the pressure differential device or block it.. do a search on that, it’s not difficult.

[APS]

If the rear brake circuit is open, ie a rear wheel cylinder valve is open , and you fill the mc reservoir, the brake pedal will be able to be pumped. Despite the front circuit being closed, until all air is removed from the rear circuit and the valves closed. Yes there is good chance of upsetting the pdwa.

Gravity bleeding however is my preferred method. It does not involve pumping the brake pedal, and does not upset the pdwa switch. Just a little patience and faith if you have not done this method before. When the brake circuits are full of fluid, with no air you get a really good firm brake pedal. It can also save the mc from being taken too far in its stroke when using the other method of pumping the pedal.

Gravity bleeding method. Fill the mc reservoir. Open wheel cylinder valve on offside and put bleeding hose on and into jar. The brake fluid will slowly start to drain down through the brake circuit to the wheel cylinder. Close valve when no more air bubbles in the hose. Do same to nearside wheel cylinder. Keep mc topped up during this process. Sometimes, especially if patience is thin, you may need to put a vacuum on the first wheel cylinder to just get the fluid to start flowing through the mc. This process has never failed me, only on cars that have abs have i needed to use a pressure bleeder. With this gravity method there is no pressure being exerted in the front circuit, the fluid just slowly drains past the pdwa switch in the rear circuit, the pdwa switch is therefore kept exactly where it started and does not require resetting. Hope this helps.

[gbb483]

Gravity filling should work, that's how the system emptied itself whilst I was rebuilding the mechanics.

[Symmo]

i use a vacuum brake bleeding kit makes life a lot easier and not a lot of money on amazon etc. works well and doesn't disturb the PWDA valve

[MandM]

I added a small bit of plumbing tape to the rear nipple threads this helps make sure that the air comes out of the bleed tube and not in or out around the loose threads of the bleed nipple.
don't go mad with the tape!
you can hold the the shuttle valve in position with a modified bolt have a search.


here you go only do them up finger tight, or you will possibly bend the shuttle
IMG_20180419_180158_HDR.jpg

[gbb483]

I added a small bit of plumbing tape to the rear nipple threads

Wish I had thought of that - the air pours in even when the nipple is only half a turn open.

you can hold the the shuttle valve in

You've lost me - where and what is the shuttle valve?

[gbb483]

You've lost me - where and what is the shuttle valve?

OK The PDWA device, replace the switch sensor with a bolt and lock the shuttle in the central location.

[MandM]

the bolt is not hard for someone with a lathe to knock up for you, then bleeding as normal no problem going forward for your ownership

[sujitroy]

I have a YouTube video on the pdwa. and a simple way to keep that thing centered

[gbb483]

I don't know why I was worried about the PDWA, a PO obviously wasn't. I've just discovered that they had disconnected the electrical plug and left it like that.

[MandM]

that just makes the light come to let your know it's been activated, so shuttle may be in the wrong place

[Chris01573]

I just did mine and use a pressure bleeder, I had a spare brake fluid cap so fitted one of the pressure bleeder fittings into it,, then just pumped up the bleeder to about 5 psi (any higher and i was worried the lid might leak). I took a 5/16" bolt and tapered the end to a point then just finger tightened this into the PDWA to lock the shuttle. Worked a treat.