Hi again folks.
Ok, I suppose you’re wondering “I thought he’d finished that little job ?” and in one sense you’re right. The car has been running with rear discs for over a month now, and I’ve been slowly tackling the niggles as I came across them.
The first was what I considered to be excessive travel on the pedal, and the initial attempts caused me to fit 2psi residual pressure valves to both circuits which rather surprisingly did make a difference but the pedal travel still didn’t match the original calculations. It was around 1/2” too much, which doesn’t sound worth bothering with but was bugging me. With full application the brake was noticeably below the accelerator pedal which it hadn't been before, so it boils down to "if it feels wrong, it probably is wrong" - a confidence thing.
I was going in circles so I moved on to the next job, which was to fit a flexible pipe just before the calipers so that I could swing the sliding component up to replace pads in the future rather than dismantle the whole system. There’s nothing significant here other than selecting the shortest flexible I could find which will allow enough movement. In normal service it doesn’t move or vibrate and just acts like a fixed line.
But it was during this stage that I finally solved the pedal travel problem and as usual, it wasn’t rocket science it was something I should have seen earlier but had missed.
You may remember I’d switched the inlet & bleed ports round because the caliper was designed to mount as vertically, but to get it on the Europa it needed to be either vertical leading or horizontal due to the spring/damper assembly. The handbrake meant horizontal mounting which placed the inlet at the top with the bleed nipple below. Clearly a problem for bleeding, so I’d just checked both holes had tapered seats and swopped them over as shown in the attached photo.
And that was mistake #1 (actually it’s more like mistake #101 ! )
Now that I had the calipers on the bench working out the new flexible connections I finally realized that they’re actually different sized ports, and doing the go/no-go test with some drill shanks I reckon the original inlet is 3mm with the bleed port 1mm less. Yep, should have seen that before, but I didn't.
From my 40yr old physics I don’t think this would affect the pressure behind the caliper piston & hence clamping force on the disc, but it will affect the rate of flow into the caliper as the smaller hole is acting as an orifice plate, and hence speed of application. A marginal effect I’m sure, but not knowing enough about the physics I’m left thinking that the designer must have picked differing sizes for a reason otherwise why drill 2 different sized holes ?
So I reversed the ports to OEM, which meant that to bleed the system properly I needed the calipers partially dismounted and moving on the (new) flexible hoses. More hassle....
But hey, guess what ? I started with the caliper static and the bleed nipple uppermost, but for some reason I decided to manipulate it during the process and was rewarded by another slug of air bubbles coming out. I can only guess that previously swopping the ports in combination with the mounting position, I’d somehow managed to lock some air inside the calipers which didn’t come out with normal bleeding. But surely the excess travel couldn't be something as simple as a bubble of trapped air ?
But it was you know. The end result is that travel at the master cyinder stroke has dropped by over 5mm and total pedal movement is now roughly 1” from start of application to full lock-up. Maybe a touch more because it’s difficult to measure accurately down the footwell, but it’s significantly less than previously and a great confidence boost.
All that trouble over a bubble of air.... just like it says in all the books. Who'd have thought....
