Brake Pedal Low Pressure

[boycer]

Hi,
Fitted new pistons & seals to 944 callipers. Callipers were off for a few weeks, so flexible hose was clamped.
Refitted callipers & bled brakes, but now can only get pressure on pedal after 5-6 pumps on pedal.
When pressure is there, when holding pedal down, pressure remains & the pedal doesn’t drop to floor.
Bled brakes 4-5 times now, but still can’t get any real pressure.
No leaks, but was a slight bit of fluid where piston goes into brake master cylinder, but wiped it off & hasn’t cone back, even after several days including lots of pumping.
Anybody any ideas why I can’t get the pressure up on the brake pedal.
Thanks

 

[Coda]

To me it sounds like there's some air still in there somewhere.

 

[fallingoffalot]

Did mine recently - new pistons etc - I completely emptied my master at one point and it took a few attempts to get all the air out. Did it the traditional way with my lad pumping the pedal.

The final attempt was successful when I bled the brakes with the front jacked up to make access easier.

I did read somewhere on the Samba that you should park your van on a hill when bleeding the master cylinder (avoiding the need to take it out and bench bleed), it didn't say which way though. Worth a try!

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[Moseley]

Hmm, any fluid around the master cylinder is not good as if fluid can come out, then certainly air can come in. However, your symptoms do sound more like air still trapped in the system. A normal sign that your MC is dead is that when pumping your pedal to pressurise the system, it will slowly sink to the floor as the fluid passes the seals back to the reservoir.

What are you getting out of the bleed nipples when you bleed the callipers? Are you getting a good rush of fluid with each pump, is there any signs of air?

 

[sparkywig]

There's still air in the system.

 

[ozziedog]

I’ll go along with the masses here and say that it seems there’s still air in there somewhere. The jacking up the front or parking on a steep drive facing up sounds like a proven recipe for success. Are you using a tube into a bottle to catch your bled fluid? Might be worth paying a little bit of attention to keeping that below fluid level so air can’t get back in plus you can often see air bubbles going down the tube when bleeding. I’m still a fan of the gunson slightly pressurised method but some don’t get on with it. The main reason I’m saying about air, is if you pump it , then you get reasonable pedal and that usually points to air in there somewhere. Another possibility is are they a matched pair of callipers? Are the bleed nips at the top? :mrgreen:

Ozziedog,,,,,,,,,sounds challenging to say the least. :mrgreen:

 

[boycer]

Thanks for the replies.
I’ve actually got the van raised at the front. When I’ve bled it I have had bit of both, at first there was a big rush of air & now hardly any air & obviously none when I stopped on each wheel.

I’ll try again tomorrow & see if I can get any more pressure.

Thanks again

 

[ozziedog]

Keep your eye on the tube from the bleed nip, and keep it below the level in the jar or container that the old stuff is going in to. If you are ‘’nose up’’ and bleeding until there is no visible air coming out ( in the tube) then as unlikely as you are thinking, air must be getting back in somewhere. Before you bleed again, actually take out each nip and clean and inspect it, also give the seat a good once over. If you have a phone device or iPad, take pictures of the nip and it’s seat then you can expand them and give it a good once over. Might have a look at where your flexis were clamped for damage, if they’re marked, wrap a load of tape around the marks, masking tape is good and you’ll soon see if there’s any leak there, or kitchen roll wrapped tight and sellotaped on.
I’m about to have these pleasures myself as I got my front hubs off and I’m trying not to disturb my hydraulics, drums can be even more of a pain. :shock:

Ozziedog,,,,,,,,,,, you are bleeding the entire system aren’t you? :mrgreen:

 

[Moseley]

What technique are you using for bleeding? There is a mix of opinions on this, my personal preference is the old fashioned two-person method. If your problem is just a bit of residual air stuck in the system, the stop-start movement of brake fluid using this method will help to drag air bubbles along to the bleed nipple. Make sure that when you get clean fluid at the nip, you do a good few more cycles to ensure that there’s no more air behind it. Sometimes it helps to leave it overnight and then re-bleed as this will help the air bubbles to settle and coalesce, so you might have more joy when trying again.

If you can’t stop seeing air bubbles at the nipple, the system is pulling in air, and I would suggest this will be your master cylinder, especially as you’ve found traces of fluid here.

I’m not sure of the logic of bleeding with the front jacked up. Whilst bleeding, the air will always go to the highest point of wherever it is sitting. Given the path from the MC of the lines, any air downstream of the MC will always remain in the lines, or in the calliper. You want to get the fluid moving nice and quick so that the air can be pulled to the calliper, rather than sitting in the line and consequently floating back to the high point.


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[boycer]

Ok, so bled the brakes again today. First tried it with the eezibleed thing, which kept the pressure up ok, but didnt suck any fluid from the bottle! A little air came out, and the pedal pressure did feel better, but not normal. Then bled it manually loads of air came out, but pedal pressure was worse! After finishing pumping the brake notice lots of air coming back into the top up resevoir and the fluid level rose.
Bit lost now, no sign of leaks or anything obvious. Feel if I could get the eezibleed to draw fluid from the bottle that comes with it, it would probably do it ok (I have bled the brakes before with it & it worked ok?!)

Anyway any further help or thoughts would be appreciated!

Cheers

 

[Moseley]

I’d say your master cylinder is pulling in air. Aside from my previous comments regarding you seeing fluid around it, if the pedal gets worse when bleeding the old fashioned way, it’s likely that the movement of the MC plunger is allowing air in. When bleeding using the pressure bleeder, you’re not moving the plunger, so the seals might be doing a better job. The danger being that if this is the case, you might eventually get a good pedal feel, but after driving and using the brakes, the system will pull air in again. Also, if it were just trapped air in the system, the fluid level in the reservoir would drop as the air is displaced by fluid. If you saw it rise, then something other than the brake fluid is being introduced to the system.

 

[fallingoffalot]

Sounds like you were pushing more air in the system with the eezibleed, if loads came out when doing it manually, that would suggest this technique is working. I didn't have much luck with the eesibleed thing and exploded my top reservoir!

You could always get a new MC, I was contemplating it when it seemed like I couldn't get mine sorted. There are plenty of YouTube vids on changing it - looks quite straight forward, although a bit fiddly!

I guess another option is to take it to a mechanic, pref. an AirCooled man, at least they could tell you if it's shagged or not?

 

[gagvanman]

This might sound stupid, but have you got each calliper on the correct side ie with the bleed nipple on the top.
I don't know if it is possible to get them the wrong way round, but I did once when I built a kit car a few years ago.

 

[boycer]

Hi gagvanman, the bleed nipples are the bottom of the calliper, but I don’t think I have mixed up the callipers, does that make a difference?

I’ve swapped out the old master cylinder and put in a new one and it definitely seems better, but still no pressure on the first push of the pedal.

Ive bled the brakes till there is no air coming out, but still no pressure on the first push of the pedal. Could it be the calliper thing, or something else?

Cheers

 

[sparkywig]

The bleed nipples should be at the top of the caliper otherwise air is trapped in the caliper above the bleed level.

 

[gagvanman]

As Sparkywig says, bleed nipples at the top.