double pump to get good brakes??

[tedtaylor]

OK, i'm no amateur, but not an expert either. I rebuilt the brake system on my Elan Sprint (rebuilt master cylinder, all calipers w/ stainless steel pistons and new seals, new pads all around) There is NO booster and never equipped with one. The only modification is larger front rotors with Girling 16P big calipers - from a Plus 2??

I've bled the system from rear first to front last. There are no fluid leaks and pads and rotors are mated and wearing evenly. The car does not pull to either side, but straight and steady. The brakes work and stop the car.....BUT the pedal goes down far and great effort is needed to stop car quickly. Now if i double pump the pedal, the 2nd pump is higher off the floor and stops the car much more effectively. So for good braking i need to double pump. Now i would much prefer the feel and results of my 2nd pump to occur when i apply the brakes the first time.

I'm thinking it must be air still in the system??? Can it be anything else?? what do you suggest i do to fix this.
Very frustrated!!!
Thanks!

[bitsobrits]

Larger calipers are likely going to take more total fluid movement to fully actuate than the stock calipers. More fluid movement equals more pedal travel. A larger master cylinder will reduce the pedal travel, but now your applied force goes down, so more effort is required. Which means you need to look at increasing your mechanical (pedal) ratio to compensate for the reduction of hydraulic ratio. A certain "chasing of your tail" applies.

When you start changing around braking components, you need to consider the whole braking system and be prepared to make many mods to retain a factory feel to the pedal.


Or you just still have air in the lines

[tedtaylor]

thanks for that....
i actually have a stock pair of OEM calipers I guess i can rebuild and install, but i think the rotors will also have to be changed out, correct?? I think the rotors i have now are thicker and larger diameter from a Plus2?? Not sure...

[StressCraxx]

Hello Ted,

Bits o' Brits is correct.

Several things to look at:

As your pedal travels towards the floor, you lose mechanical advantage. You fill the front calipers with the first pump, then refill the master with the second pump and you regain most of your mechancial advantage.

Have someone pump your brake pedal and look for a turbulence in the reservoir. Turbulence means you still have air in the system.

Very carefully check your wheel bearings for looseness and rotors for runout. Either will make for brake pad "knock back". The pads and pistons get pushed back and you need two pumps to refill the calipers and bring the pedal back up.

Regards,
Dan

[tedtaylor]

Not trying to argue, but these are brand new THICK pads. The pads are floating against the rotors with the caliper piston against the pads and fluid already in place behind the piston. I cannot understand your point. You make it sound like the pistons retract completely back into the caliper, but that is not the case as they are in slight contact, ready for the fluid compression sent by the pedal action. There is minimal movement. The only thing that would move or take up the slack would be air or weak rubber brake lines? My lines are OEM rubber (not updated metal encased lines). I will certainly attempt to pressure bleed them again, but if it is not air and not expanding rubber brake lines, and no movement or slack in the setup, then I cannot follow your train of thought.
Sorry.

[mbell]

I am with Dan, you need to check the end float on your hubs/wheel bearings. If they are lose the hubs have side to side movement that knocks the pads away from the disc as you drive.

First time you less the pedal you have to move the pads back to the disc, which requires more pedal travel. Second time pads are still on disc so shorter travel.

Checking/adjusting this is next in my to-do list as I know I have, some probably too much, end float now things have needed in a bit.

[vxah]

I found the same issue after a calliper rebuild, I found that if the front pistons were pushed all the way back and then the pads fitted I had to "double pump" the brake pedal. If I took the pads out one at a time and pressed the pedal to push the piston out then pushed the piston back in just enough to get the pad back in all was fine (after all were done)
I have put this down to the piston seals? I think that if the pistons are a long way back the seal tends to roll over in the groove when the piston slides over it, when the brake is released, after a time the seal rolls back pushing the piston back that little way with it? So when you push the piston back a little way to fit the pad the seal is tending to want to keep the piston outwards.. Might be the aftermarket seals not being quite right?

[rgh0]

Most of the common possible reasons have been covered already

- still air in system - bleed more
- excess piston pull back by the seals - try some rubber grease on the seals
- disk run out and pad knock off - check and replace disks if excessive
- loose wheel bearings and pad knock off - adjust

other potential issues that may cause the problem

- excess free play in master cylinder push rod and pedal assembly
- reservoir seal at the end of the master cylinder not seating correctly
- brake pads not seating square versus the disk ( but unlikely as you see they are bedded in evenly)


I would carefully watch pad movement with the wheel off when you first push the brake pedal down and then on the second pump and observe what is actually happening.

cheers
Rohan

[tedtaylor]

all good (and very technical) points raised, thank you.
I'll double check another day soon, but i did check the hub for "end float" or "end play" and there is no movement, snug, so i feel pretty good about the wheel bearings/hub are not moving side to side.
I'll check it all and report back.....when my back feels better!

[fattogatto]

Have you bled the master cylinder itself?

You mention that you bleed rear to front. Try bleeding all four corners at the same time (yes, I know it requires help - or the use of speed bleeders, which work quite well.) After you bleed the master cylinder. If you still have issues then start isolating each wheel by capping off the brake line. With all four brake lines capped you should have a rock solid pedal with no movement. If you have movement then its the master. If it is rock solid, start re-attaching brake lines one at a time; bleed and check pedal. This should narrow the focus.

[vxah]

Buy a pair of brake hose clamps (little L shaped things with wing nuts) clamp the front hoses and see what the pedal is like? Remove the clamps one at a time and see what happens?

[Orsom Weels]

Had exactly this problem on my +2 after a servo rebuild a few years ago. I pressure bled the system countless times, never getting any more air out, but still ended up needing a second pump on the pedal, the first went nearly to the floor. Tried all the things suggested here, all to no avail. In the end, when I had an assistant handy, I went back to the good old fashioned way of bleeding, with mate in car & me on one bleed screw at a time. Open bleed screw, get mate to push pedal down, lock off bleed screw & get mate to release pedal etc. After about half a dozen pumps, I got a fair bit of air from all four lines. After that, I had a rock solid brake pedal & it's been fine ever since. Why the pressure bleeder wouldn't shift the air I don't know, & I know I have a servo in the system which you don't, but might be worth a try if you haven't already.

Tim

[Sking06]

I have the same problem with my '69 Elan. I have rebuilt the brakes a number of times in the last 48 years and have never had this problem. The only difference this last time, besides brake fluid, is the chrome pistons. I think the piston seal is doing what it is designed to do, pulling back the piston, but is doing it to well and that it may have something to do with the chrome. When I apply and release the brakes with the wheels removed, I can see that some of the pads have more clearance than others.

[RichardHawkins]

Ted,

I friend had a similar problem with his +2.

Using the traditional method with an assistant he still had a soft pedal, but no more air came out.

In his case the eventual solution was embarrassingly simple. The brake pedal was being depressed too slowly. When he changed technique, to "kicking" the pedal down he got more air out and his brakes have been fine ever since.

It appeared that depressing the pedal gently did not create enough velocity to carry the air forward with the fluid.

I think I remember someone about 40 years ago (Lotus Europa) who removed the calliper to bleed it because the bleed screw was not at the high point!

Richard Hawkins

[collins_dan]

Ted, I had a similar problem on the S4. First check, if you hold the pedal down for a minute or so, does it gradually sink down any further? Mine did because I assembled one of the seals in the master cylinder backwards. It's an easy thing to check. Good Luck, Dan