mn7250 Posted August 15, 2009 Share Posted August 15, 2009 Hoping for some pearls of wisdom here, after experiencing lack of braking power on hill descent. Vehicle is a 300Tdi 'M reg, non ABS. Descending a fairly steep hill, brakes decided they'd become pretty much ineffective. Pedal travelling usual amount but virtually no stopping power. Managed to come to a halt at the bottom by standing on pedal, and pulled over. After much head scratching, started engine up again, and brakes back to normal. Managed to drive back home okay, avoiding steep descents, and brakes still fine when we got back. Had the same problem back at Easter, and thought I'd boiled the fluid or maybe air was in the system, so we did a complete fluid change, switched the flexible hoses to new, braided varieties, and also fitted a secondhand vacuum pump, servo and master cylnder. NO problems at all on a trip to Cornwall (lots of hills) a few weeks ago, but now the problem seems to be back. What am I missing? I've just run the car up, and pulled the vacuum pipe off at the servo end, and there's definitely a strong vacuum pulling my thumb onto the pipe end. Done the usual "pump the brake pedal before I start up" test; with foot on pedal and then starting the engine, my foot moves down a little, so I'm pretty certain the vaccum pump is working. Is it possible for the servo to leak and after prolonged braking (the hills in each case were very steep) I lost all vacuum assistance? Would a cracked pipe from the pump to the servo cause this leak, or is the NR valve in the servo supposed to stop this? It's so odd that normal braking resumes as soon as the descent is over and I can't seem to replicate the fault by driving then braking continually. My money is on the servo being the culprit,and I'll probably end up buying a service kit for it for starters, but if anyone else has any other ideas, I'd greatly appreciate them. Quote Link to comment Share on other sites More sharing options...
pinny Posted August 15, 2009 Share Posted August 15, 2009 hello i know its clutching at straws have you checked your wheel bearings if thay are loose it will cause the pads to move away from the discs causing more travel when braking you wouldnt notice this if you was breaking every 300/400 yards but over a distance it will push the pads away worth a look,chris. Quote Link to comment Share on other sites More sharing options...
matt bristol Posted August 15, 2009 Share Posted August 15, 2009 If you still have a normal length of travel on the pedal my money would be on overheating the pads, this causes the glue holding the friction material to melt and lubricate the pads against the disc, once it cools down again they work again. I think the ratio of lost performance is something like 50% less effective for the first 20% of pad wear, so new pads will help but if you are using the brakes a lot or for an extended periodf eg big descent tehn it may still happen. Quote Link to comment Share on other sites More sharing options...
WESTENDER Posted August 15, 2009 Share Posted August 15, 2009 if your servo stops working you dont get the usual amount of pedal travel [a lot less]and a very hard pedal it sounds like you had brake fade after a long steep descent, get some decent pads and check your discs are within wear limits, minimum thickness is usualy marked on them somewhere if the discs are excessively worn they will heat up very quickly and your brakes will fade Quote Link to comment Share on other sites More sharing options...
SPendrey Posted August 15, 2009 Share Posted August 15, 2009 That's interesting, I've had exactly the same sort of problem of late... although it doesn't always happen. I have got a squeal at the moment too, more-or-less constant and I think from the front axle somewhere. All the pads and discs are less than 1 yr old. The only difference is that I have ABS, but most of the time it tells me it isn't working! Quote Link to comment Share on other sites More sharing options...
mn7250 Posted August 16, 2009 Author Share Posted August 16, 2009 Thanks for the replies so far. Front pads were replaced when I did the other work after the first failure - I put a set of Mintex pads on which have always served me well in the past. Rear pads were only 20% worn at most. Front and rear discs themselves were replaced less than two years ago and I've only done about 10000 miles on them and they show little wear. Rear calipers and pads are also only two years old - calipers had worn to the extent that the pads had dropped through and started to grind their way into the disc hub! I checked the front calipers when I changed the pads recently and had to replace seals and pistons on the nearside as they were not retracting. Haven't had any pulling to one side, so I assume that they are both now working okay. Wheel bearings were checked also and were fine. Bit difficult to say whether the pedal travel had decreased at the time it happened - I was coming down a narrow single track road off a Welsh mountain and only started to notice the brakes weren't stopping the car gradually. Pedal pressure was VERY hard. Have had a look at the owners manual and it would appear that the only serviceable part for the servo is the filter assembly, so I may be looking at replacing the servo again (but this time a new one). I'm going to have a chat with my local Landrover specialist tomorrow, so I'll let you know what he says. Quote Link to comment Share on other sites More sharing options...
mn7250 Posted August 16, 2009 Author Share Posted August 16, 2009 Just another thought on this, and still hoping that someone might have the knowledge.... What if my EGR valve has failed and is going fully open when I'm coming down hills? The manual says it kicks in between 630 and 2850 revs, which corresponds nicely with me coming down a hill on overrun and under load. As this is vacuum controlled via the T-piece on the vacuum feed to the brake servo, could this be robbing the servo of the vacuum needed if the valve is wrecked inside and is draining the vacuum? Wuold also explain the return to normal after stopping as the EGR shuts off after 25-30 seconds idling. Is this a brainwave or am I clutching at straws.....? Quote Link to comment Share on other sites More sharing options...
SPendrey Posted August 23, 2009 Share Posted August 23, 2009 OK, so I've checked the pads and discs, all good. The brake pistons are all mobile too, so they are out of the equation. I'm down to either replacing the vacuum pump or the servo now. My question is which first? I know there is a vacuum because if I wiggle the one-way valve (which is also fine by the way) at the servo I hear the vacuum release slightly. any suggestions which I should replace first? Thanks. Quote Link to comment Share on other sites More sharing options...
WESTENDER Posted August 25, 2009 Share Posted August 25, 2009 surely if you have got a vacuum your pump is working? so why change it? the servo is easy to test,with the engine running press on the brake pedal shut the engine off and repeatedly press the pedal, travel will reduce drastically until you cannot move it much at all, restart the engine with your foot on the pedal and it should fall away under your foot indicating servo assistance. Going back to your original post, having been a lorry driver for more years than i want to remember mostly tankers with unbaffled tanks i think that old adage of "go down a hill in the gear and at the speed you would go up" still holds a lot of truth especialy with standard brakes on a heavy vehicle Quote Link to comment Share on other sites More sharing options...
SPendrey Posted August 25, 2009 Share Posted August 25, 2009 Well, you're right 'westender', the current pump is pulling ~24inches mercury (which seems OK, a 200TDI we had access to did ~27). So the vacuum us good. The hoses are all sound too. The servo test appears correct too, so now we suspect the master cylinder. There is a little creep when constantly applying pressure to the pedal, but less than on the 200TDI Defender So, now we plan to bleed and flush the brake fluid just in case, and since we're doing that we'll try another master cylinder (hoping to find a used one locally in order to eliminate it from the equation without costing an arm and a leg for now reason). The good news is that the front left wheel/hub had a little play in it. That's all tightened up and now the ABS light isn't permanently on! Thanks to 'Countax' for his help there! Quote Link to comment Share on other sites More sharing options...
errol209 Posted August 26, 2009 Share Posted August 26, 2009 If you have a good hard pedal with normal travel and it stops eventually, a good vaccuum and all the mechanicals are good then it can only be the servo. Mine didn't do the sinking-when-you-start thing, so I took the servo off, re-connected the vaccuum line and started up - you'd expect the servo to "happen" when you push the yoke end (like pressing the pedal) but nothing. My conclusion is that the valves have got bunged up, so the servo isn't allowing atmosphere in. This must have been a gradual process, as the brakes have never been what you might call "sharp". New servo on order ... Quote Link to comment Share on other sites More sharing options...
SPendrey Posted August 26, 2009 Share Posted August 26, 2009 I think you're right, but I can't conclusively prove it. Tonight we replaced the master cylinder and servo (as they were already joined together), as well as a full brake bleed. Problem gone! Quote Link to comment Share on other sites More sharing options...
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