Filbee Posted March 24, 2022 Share Posted March 24, 2022 I'm a bit stumped at the moment... I've just fitted some brand new AP front calipers with stainless pistons to my 2006 Td5, non-ABS 110, as the pistons in the original calipers had rusted and partially seized. I got them fitted with no issues, popped in some new pads and got the better half to work the brake pedal while I bled them. I got a good bleed, all the air bubbles soon disappeared and the pedal felt quite firm. That is, until I tried driving it. Once vacuum was working the servo, the pedal just steadily sunk lower and lower... I've checked all the pipe unions, bleed nipples, pulled the pads back out to check there are no leaks from the piston seals, and run my hands around the master cylinder and calipers. It all appears dry and fluid-tight. The fluid level in the reservoir didn't seem to drop at all. Having read a few posts on here about sinking brake pedals, the most logical culprit seemed to be a goosed master cylinder. I thought maybe the seals had given up after the brake bleeding exercise. I've now fitted a NOS Lucas master cylinder, and it's made no difference 😟 I filled up the reservoir with fresh fluid and made sure it was coming out of all three ports before I fitted it, and then pressure bled the system with a Gunson pressure bleeder (the one you hook up to a spare tyre), and again, it all seemed to bleed nicely. With the engine off, the pedal seemed fairly firm, but once it was running, down it went. I checked for leaks again and found none. The fluid level is steady. I'm all out of ideas now, so I wondered if anyone has any suggestions as to what I do next? For now, I've left it with a piece of timber jammed between the seatbox and brake pedal to give any air bubbles a chance to work back up to the master cylinder. Quote Link to comment Share on other sites More sharing options...
western Posted March 24, 2022 Share Posted March 24, 2022 Are the caliper bleed screws at the top? If not you will never get then bled properly. Examine the servo around the studs for hairline crackers & corrosion. Quote Link to comment Share on other sites More sharing options...
Filbee Posted March 24, 2022 Author Share Posted March 24, 2022 Yes, bleed nipples at the top! I gave the servo a good clean before refitting the master cylinder and it all looked in very good shape. Out of curiosity, if the servo was damaged, would that make the pedal sink to the floor? I always thought a knackered servo had the opposite effect and made the pedal much harder to push. Quote Link to comment Share on other sites More sharing options...
Eightpot Posted March 24, 2022 Share Posted March 24, 2022 You might have damaged the old seals in the master when bleeding, and now you may have an airlock in the new master. Some masters need to be primed. Also you may need to do a full 4 wheel bleed if you haven't already. Quote Link to comment Share on other sites More sharing options...
Filbee Posted March 24, 2022 Author Share Posted March 24, 2022 (edited) 16 minutes ago, Eightpot said: You might have damaged the old seals in the master when bleeding, and now you may have an airlock in the new master. Some masters need to be primed. Also you may need to do a full 4 wheel bleed if you haven't already. I primed the master by filling the reservoir and making sure fluid was coming out of all three ports. And I've bled all four wheels more than once 👍 Edited March 24, 2022 by Filbee Quote Link to comment Share on other sites More sharing options...
western Posted March 24, 2022 Share Posted March 24, 2022 If its cracked then you won't have any vacuum assist when driving. But sounds like there is a big airlock somewhere or the master cylinder is faulty. Quote Link to comment Share on other sites More sharing options...
Filbee Posted March 25, 2022 Author Share Posted March 25, 2022 7 hours ago, western said: If its cracked then you won't have any vacuum assist when driving. But sounds like there is a big airlock somewhere or the master cylinder is faulty. Vacuum assist is working fine. I think I'd be pretty unlucky to have two duff master cylinders. The problem is exactly the same after fitting a brand new Lucas m/c. It's not impossible, but surely it's pretty unlikely? Which leaves potentially an airlock. I primed the new m/c before fitting it to minimise the chances of that happening and used a pressure bleeder on my second attempt, and it seemed to bleed really well. I'll have one more go today and flush another litre of brake fluid through. Quote Link to comment Share on other sites More sharing options...
Maverik Posted March 25, 2022 Share Posted March 25, 2022 Have you driven it? To feel what is like under foot under normal driving conditions? I say this as I remeber having this symptom with a D1, when I actually went to drive it, the pedal sink didn't effect the feel or braking ability and you didn't actually notice it. To me the physics didn't make sense, but it worked ok. Try bleeding with the engine running? Quote Link to comment Share on other sites More sharing options...
missingsid Posted March 25, 2022 Share Posted March 25, 2022 With the engine off does the pedal return to normal feel and height when pushed down? On a servo assist car if you press the brake pedal and hold it during starting the engine the pedal will always drop further. If you stop the engine the pedal should IIRC push back at your foot as you are making all the effort needed. How much travel do you have before the pedal goes hard under foot with no servo assist, maybe just too much play? Quote Link to comment Share on other sites More sharing options...
Filbee Posted March 25, 2022 Author Share Posted March 25, 2022 Thanks all for keeping the suggestions coming 👍 I've done another round of bleeding, and there were no bubbles coming out at all. I decided to just go for a chug round the field to what happened. She will stop with a good shove of the pedal, enough to lock at least one of the wheels (on grass!) but the pedal travel is much longer than it was before I changed the calipers and it feels a bit soft. When I came out this morning after having the pedal wedged down over night there was a bit of fluid on the very bottom of both front calipers. Just a smear, not a drip. I took the pads out again and there's no evidence that it's come from the piston seals. I guess I'll just drive around steadily for a bit and see if things get any worse! Quote Link to comment Share on other sites More sharing options...
cackshifter Posted March 25, 2022 Share Posted March 25, 2022 I have a similar problem, just rebuilt all 4 calipers, new pads and bled everything and the pedal is all spongy. One thing it can be is the back of the pads sometimes has a squishy anti-squeal layer on it, and until the piston has bedded into that there is a bit of give which is magnified at the pedal. I can lock wheels but the pedal feels awful, contemplating a new master cylinder but like yours, it seemed fine before. You are not alone! In fact I suppose one option is clean up the old pads and put them back. If a hard pedal returns we know what it is. Quote Link to comment Share on other sites More sharing options...
Filbee Posted March 25, 2022 Author Share Posted March 25, 2022 4 hours ago, cackshifter said: I have a similar problem, just rebuilt all 4 calipers, new pads and bled everything and the pedal is all spongy. One thing it can be is the back of the pads sometimes has a squishy anti-squeal layer on it, and until the piston has bedded into that there is a bit of give which is magnified at the pedal. I can lock wheels but the pedal feels awful, contemplating a new master cylinder but like yours, it seemed fine before. You are not alone! In fact I suppose one option is clean up the old pads and put them back. If a hard pedal returns we know what it is. Your issue sounds exactly the same as mine. I can rule out squishy pad backing material though - my Ferodo pads don't have any... Quote Link to comment Share on other sites More sharing options...
Mo Murphy Posted March 25, 2022 Share Posted March 25, 2022 Fil, I would drive it for a week, I think you'll find the feel of the brakes will improve as the pads bed in. HTH Mo Quote Link to comment Share on other sites More sharing options...
Filbee Posted March 26, 2022 Author Share Posted March 26, 2022 9 hours ago, Mo Murphy said: Fil, I would drive it for a week, I think you'll find the feel of the brakes will improve as the pads bed in. HTH Mo Thanks Mo. On the basis that she does come to a stop and it seems to be more of a pedal travel/ feel issue, I'm happy enough to drive around for a week and see how it goes👍 Quote Link to comment Share on other sites More sharing options...
cackshifter Posted March 26, 2022 Share Posted March 26, 2022 I can report, I went for a drive,and it felt awful, but stopped fine. I left it overnight, bled again, and drove again and it felt much better. I have just put LOF pads in, the ones I took out were Ferodo, and both have an antisqueal coating on the back of the pad. When you think, a very small movement at the pistons is highly magnified at the pedal ( the whole point of hydraulics)it isn't so surprising. On the second drive, the pedal felt much better. I am going to leave it and see what happens, it certainly stops ok, it's just the feel. 1 Quote Link to comment Share on other sites More sharing options...
Filbee Posted April 3, 2022 Author Share Posted April 3, 2022 Time for an update a week after fitting my calipers... I've not done many miles but the slight weep of fluid found under the calipers seems to have stopped, which is a relief, and whilst the pedal still doesn't feel as firm as it once did, and travels further, it doesn't creep down to the stop any more if you keep pushing it, and she does pull up smartly when required. I've got some braided hoses to fit at some point so will report back again once fitted as to whether they have firmed up the pedal feel at all 1 Quote Link to comment Share on other sites More sharing options...
defender dinky Posted April 3, 2022 Share Posted April 3, 2022 Hi, have you change all the calipers, if the front one's have three nipples on, the inside one's have to be bled together, had this on my 110 project which is now sold, I haven't read all the post, thought it might help, dd Quote Link to comment Share on other sites More sharing options...
Filbee Posted April 4, 2022 Author Share Posted April 4, 2022 On 4/3/2022 at 8:02 AM, defender dinky said: Hi, have you change all the calipers, if the front one's have three nipples on, the inside one's have to be bled together, had this on my 110 project which is now sold, I haven't read all the post, thought it might help, dd Thanks for the reply but no, they each have one bleed nipple, at the top of each caliper. I've never seen a caliper with 3 bleed nipples. I'm glad mine only have the usual one per corner! Quote Link to comment Share on other sites More sharing options...
chuff Posted April 23, 2022 Share Posted April 23, 2022 You can narrow down which circuit is causing problems by clamping the front or rear hoses. When bleeding a troublesome system proper ram the pedal down as fast and hard as you can, there's a good chance a big slug of air will come out. Quote Link to comment Share on other sites More sharing options...
Junglie Posted May 5, 2022 Share Posted May 5, 2022 Just a thought, and it might not explain why things are different with the engine running, but... Are the replacement calipers the same piston size? Because if the pistons are bigger, though you'll get better braking force you'll also need a bigger master cylinder because the ratio of master cylinder to slave cylinder volume will have changed...and you'll need more movement at the pedal. Quote Link to comment Share on other sites More sharing options...
Filbee Posted May 5, 2022 Author Share Posted May 5, 2022 The calipers are OEM AP/Lockheed calipers fitted with stainless pistons, which are the same diameter as the OEM calipers I took off. The brakes are working fine now, they just don't feel as firm as they once did. I guess that could be down to the fact that at least 4 of the pistons in the original calipers were rusty and seized or partially seized 🤔 1 Quote Link to comment Share on other sites More sharing options...
1750gtv Posted May 11, 2022 Share Posted May 11, 2022 My brakes didn't feel as firm after James Spurgeon replaced all discs and callipers. Definitely a case of them not working correctly before rather than now! 1 Quote Link to comment Share on other sites More sharing options...
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