Spongy brakes & Pedal box flex

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davidimurray

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Building: Haynes Roadster

posted on 23/4/13 at 09:49 PM

Spongy brakes & Pedal box flex

My car (Haynes Roadster) is yet to be IVA'd (almost ready) but I've never been happy with the feel of the brakes.

Setup is sierra master cylinder with sierra front calipers and sierra rear drums - all these are new. Only difference is that I have fitted smaller rear cylinders to reduce brake force (based on the experience of others)

When you press the pedal, the pedal gets hardish, with about 10-15mm before the bulkhead) but if you push with all your might you can just get it to touch the bulkhead.

I was slightly worried that due to the 'up and down' layout of my brake pipes at the rear that I may have trapped air. I blocked off the T-piece with bleed nipples and rebled. The pedal went stiff about 40mm from the bulkhead, but again you could push hard on the pedal and get the pedal to within 20mm of the bulkhead. Looking at the master cylinder you can actually see it flexing and moving so it looks like my pedal box plate is flexing. I checked the movement and it was only 1-1.5mm but working this back it equates to 7-10mm of travel at the pedal. I am thinking that the apparent 'sponginess' is down to the flex in the pedal box??

Based on this it looks like I need to stiffen up the pedal backplate. This is currently made from 4mm steel plate and I was thinking of making a new plate from 6mm plate.

Has anyone got any thoughts on this. Will the thicker plate be sufficient or should I also add some stiffening ribs?

I attach a couple of photos of the pedal box when I first made it a few years ago.

Cheers

Dave

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Mr Whippy

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posted on 23/4/13 at 11:36 PM

4mm plate is an overkill as it is, 6mm is way OTT (that's the kind of stuff you mount offroad rollcages too not brake pedals), think of a production car, they are lucky if the pedals are mounted onto 1mm thick panels

I recommend you buy an Gunson Easybleed kit as that is what I use for all my cars even ABS ones as the constant high flow rate sweeps out any bubbles in seconds, makes jars and pumping pedals look a right chore

Also if you gently pump the brake pedal twice and it goes down less the second time then the shoes are too far away from the drums and need manual adjusting, as most auto adjusters are useless.

halford linky

[Edited on 23/4/13 by Mr Whippy]

britishtrident

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posted on 24/4/13 at 06:44 AM

While pedal box flex is a major cause of spongey brakes using thicker plate probably won't help much, I think you need to look improving how the pedal box is tied into the main structure and improving how the pedal pivot transfers its' load to the pedal box and the main structure.

I think you may also have other problems I would check the rear self-adjusters are functioning properly also be 100% sure you don't have a weeping brake pipe joint.
A leaking brake pipe joint is not always obvious often the leak can be such that air is drawn into the system on the return stroke but there is no obvious fluid leak.

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davidimurray

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Building: Haynes Roadster

posted on 24/4/13 at 08:38 PM

Hello

Thanks for the ideas.

I had another go at the car tonight. Dug out the easibleed kit from the depths of the garage, sorted out the dodgy seals then hooked it up. Bled a full easibleed fill through the one side of the brakes connected on the rear and set the adjuster so the brakes were just dragging. I found a couple of tiny weeps on brake lines which nipped up.

I then tried the brakes again. The pedal went solid with 18mm of spare space before hitting the bulkhead. I then fitted a block between the end of the master cylinder and the engine mount.this time the pedal was again solid but had 28mm between it and the bulk head.

So it looks like further bleeding is order once I've sorted out the pedal box flex. Question is how best to do this? Current arrangement is only bolted to floor and bolted to the 1.6mm sheet across the top. I have a few ideas -
1) weld vertical strips either side of the pedal box and bolt the backplate into these
2) make l shaped brackets and weld to the backplate that hook around the chassis rails to brace the plate wen pushed
3) as option 2 but bolt brackets to backplate so no need to weld
4) weld the backplate into the car

As my car is virtually complete access for welding is very limited so currently option 2 or 3 is my preferred option.

All thoughts and ideas gratefully received.

Cheers
Dave

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