Difference between revisions of "Bleed the brakes"
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Revision as of 09:35, 25 February 2008
If air gets trapped inside the brake hoses a softer brake pedal is felt. The brakes need to be bled and the air removed from the system. This procedure is relatively simple, being made much easier with a specialist bleeder.
If you have air in the system (eg. when changing brake hoses) then you'll pretty much always need to mess around with the front callipers to get it all out as the design of them traps the air in the inner piston.
If it's just for flushing the system with new fluid (no air in the system) then it's pretty straightforward and you don't have to mess about with the callipers.
It helps a lot if you have spanners with a 'bend' at the end, like an open ring-spanner to keep the brake hose itself in place as you tighten the connector on the car side using a normal, but thin spanner.
On my (early) S2 there was enough space. On a VX220 I needed to dremel the hole above the connector a little to get enough clearance.
Procedure
Flushing system
If no bubbles are in the system and you just want to refresh the brake fluid, you can either connect a pressure bleeder or bleed manually.
Order; OSR, NSR, OSF, NSF. Wheels off for the rears, but can do it through the spokes at the front.
Manually: For each calliper in turn, pump the brake pedal until it is solid then maintain the pressure whilst the other person puts a non return value on the brake nipple and releases it (11 mm spanner) until the pedal hits the deck.
Pressure bleeder: Fill bleeder with brake fluid, connect to reservoir, pressurise bleeder. One by one open bleed valves on calipers until fluid runs clear.
Changing brake hoses
When changing brake hoses or if you have air in the system a possible approach that can save loads of time or troubleshooting when bleeding is this:
- Before changing the hoses, remove the brake pads and fit very worn/old pads (or some strips of steel or wood) in their place
- Press the brake pedal a few times to extend the brake pistons in the calipers (the old pads or wedge are there to keep them from popping out completely!)
- Now disconnect an old brake hose from the car
- Disconnect the old hose from the calliper
- Connect the new hose to the calliper and tighten up
- Have a helper keep the open hose end up as high as possible
- Gently push back the piston in the calliper (hand/finger pressure for the fronts, screw back the rear) until brake fluid just comes out the open hose end
- Now connect the hose to the fixed line on the car and tighten connection
Bleeding rear callipers
For the rear callipers the rest of the procedure is:
- Pressurise bleeder
- Tap on the new brake hose and calliper with a plastic/wood hammer to dislodge trapped bubbles
- Open the bleed valve and screw the piston back completely
- Bleed it a little more normally and check if any more air bubbles comes out
Once it's clear, close the bleed valve, refit pads and check pedal hardness after pumping a few times to seat the pads. Should be solid.
Bleeding front callipers
S1 and non-ABS S2
For the front brakes there are two ways to get rid of the small amount of air in the hose once it is re-connected to the car.
On an S1 and older S2 without ABS you can compress the pistons in the brake calliper completely. This will force the small bubble up towards the master cyclinder and out into the reservoir.
On a recent S2 with ABS this is not recommended as the air bubble could get trapped in the ABS hydraulic unit and to get it out you need to use the Lotus scan tool to set the ABS unit in a special 'flush' mode. In this case skip to the next method.
- If this worked you can bleed or flush the circuit as usual
- Close bleed valve
- Refit pads
- Test for pedal firmness.
If this worked then continue with the next calliper.
ABS equipped S2 or spongy pedal
On more recent cars with ABS, or if you still can't get a firm pedal after the first try then there's still air in this circuit somewhere and you need to bleed it the traditional way.
The approach here is a bit cumbersome because of the design of the callipers that trap air in the inner piston.
If this is the case then the approach is as follows:
- Pressurise the brake bleeder with a low (8 psi) pressure to stop fluid and air traveling upwards to the master cylinder
- Tap on the calliper and hose gently with a plastic hammer to dislodge any bubbles that adhere to the surface
- Remove a front calliper from the upright (2 allen head bolts), invert it so the connectng pipe is at the top and open the bleed nipple
- Tap the brake hoses gently and also tap on the inside of the calliper.
- Then (with your hands) squeeze the INNER piston in. This will empty the inner piston and force the bubbles to the front one. Make sure you push it completely in.
- Now carefully rotate the calliper normal side up again so the bleed nipple is at the top. Try to turn it so any bubbles in the outer piston will not try to go up the connecting pipe again.
- With the calliper 'right side up', tap the calliper again and squeeze the OUTER piston back into the calliper and watch for air bubbles escaping
- Close nipple
- Lightly bolt calliper back onto upright
- Refit pads
- Test for firm pedal after some pedal pumping to seat the pads
- If succesful then tighten calliper bolts
Conclusion
Using this method to fit new brake hoses and bleed the system the advantages are:
- You only introduce a tiny amount of air in the system, so less to get out again.
- You immediately know if you have air left in this specific part of the circuit if the brake pedal was hard beforehand, but now remains 'soft'. (no long 'hunt the problem' sessions)
Bleeders
Power bleeders
safety goggles recommended Pump up to 10psi, or 20psi... Fitted with Ø42mm cap and suitable for most European models
Eezi Bleed kit
Uses the air pressure from your tyre to force the brake fluid through.
Oh, and don't make the schoolboy error I made, when you decide that you've got all the air out then just do a one more cheeky bleed without realising that the reservoir is almost empty. Oh how you'll laugh as you shoot big globs of air into the system... :wall
Make your own with plant sprayers
use it at your own risk, but I made my own Easybleed using one of those spray can with pump included used to spray indoor plants. worked a treat (as long as there was fluid inside)
