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MG MGB GT V8 Factory Originals Technical - Soft pedal

Need some help and hoping the conversion crowd can offer some suggestions on where to go next. I have a 79 MGB with Ted Lathrups front coil over suspension with Wilwood calipers and a Hoyle rear IRS with what I believe are Ford Sierra rear disk brakes. Just finished a compete engine bay redo which includes new SS hard brake lines, SS braided flex lines, a new MC and professionally rebuilt brake servo. Despite standard bleeding procedure and reverse bleeding from the disk themselves I still have a relatively soft pedal for the first 2/3 to 3/4 travel. The car will stop but not in a way the gives the driver great confidence. Any suggestions would be greatly appreciated.
Frank Mooring

I had trouble bleeding the rear Sierra discs, and ended up taking the calipers off and holding them so that the bleed screw was right at the highest point. That did the trick. But I agree with you about the softness of the pedal. I have the complete Hoyle set up front and rear, with his 4-pot calipers up front. My brakes are fine and pull you up without drama, but feel soft as you describe. There isn't the initial bite that you get with modern over-servoed brakes. I wonder if its the green stuff pads that John Hoyle supplies, and I would like to try something else, maybe yellow stuff.
Mike Howlett

Mike,
Thanks for the reply. I did exactly as you suggested when I first installed the rear IRS and it helped but had forgotten about this when I redid all the brake lines. Certainly worth a try. I have chased this problem of a stopping car but relatively soft pedal back when I installed all the suspension upgrades and although I got it tolerable, like you it was never a truly firm pedal like I am used to on other cars. Was wondering if the MC perhaps doesn't move enough fluid to account for the brake upgrades. Anyone else with thoughts to offer.
Frank
Frank Mooring

BTW, Ted Lathrup the designer of my front suspension initially recommended installation of residual pressure valves on all three lines near the MC. In another forum I asked a similar question to the one I asked on this thread and the reponse from one poster was to remove the residual pressure valve in my MC as it(my MC) was designed for drum brakes on the rear. I don't know enough about the MC on my late model MGB to know if it has an internal residual pressure valve. I am somewhat at a loss.
Frank Mooring

I'm having the same dilemna with my V8 and talked with Ted Lathrop, Bill Guzman, and Bill Young.

(I've got Ted's front end and Guzman's four-link rear suspension.)

I'm running a stock MGB brake m/c with servo; a Wilwood proportioning valve; Wilwood four-pot calipers.

Ted recommends the installation of residual valves and elimination of the proportioning valve.

Bill G. recommends changing out the brake m/c to a GM style with a GM proportioning valve.

Bill Y. recommends same as Bill G.

I'm not sure which way to go...Ted does feel that the residual valves will cure the problem at the lowest expense.
Rick Ingram

Rick,
If you read the Wilwood website on their residual pressure valves the 2 lb ones are for disc brakes on cars with the MC in a lower position than stock (i.e. to keep up the pressure gravity would normally have) and the 10 lb ones are for cars that have drums on the rears (they require greater pressure) which is why I can't see why these would be required on my car with the stock position MC. If you work out the details on the GM style conversion I would love to hear from you to do the same. I hated cutting my brand new SS brake lines to install the residual pressure valves especially if it wasn't going to work and it is for this reason that I have yet to take this step. Thanks
Frank
Frank Mooring

Ted recommended the 2# residuals.
Rick Ingram

We have a GM style brake m/c and proportioning valve on the Roadmaster project.,,,similar to the one attached to this post.

Also:

See: http://forum.britishv8.org/read.php?2,166,page=71

Rick Ingram

I've always had a long/soft pedal that can be pumped up, but then goes long again if left a few seconds when bleeding MGB brakes and it has always needed a second phase. First phase is basic filling and bleeding using a gunsons EeziBleed until the bubbles stop, second phase is to have someone press down hard on the pedal while I rapidly open and close each (front) calliper nipple in turn. That always blasts more air out, and after that the pedal is fine.
PaulH Solihull

Paul, often the problem of a long pedal, not a soft pedal, is caused by the rear brake cylinder area being slightly larger than the master cylinder is designed for. Need more fluid to move the pistons and therefore more pedal travel. If the pedal is "soft" and spongy feeling that's usually an air problem as you state, but it the pedal if firm when it stops but the travel is longer than you like then you're dealing with cyinder size issues. Check valves work great at minimal expense, 2# units for disc brakes and 10# units for drum brakes, make a huge difference, especially with rear drum brakes as Frank said.
Frank, there are fittins available so that you can mount the check valve directly on the output port of the master cylinder and avoid having to cut your lines, just a bit of rebending might be necessary.
B Young

Certainly a possibility if one had changed the cylinders, but not if all one is doing is bleeding. In my experience on several cars if you pump the pedal rapidly you can get it hard and short, when the only problem is air in the hydraulics, as the air takes time (a few seconds) to expand to cause a long and soft pedal again. GT cylinders are actually larger diameter than all but the earliest roadster cylinders, but don't have this effect, they would need to be quite a bit bigger.

Not sure what you mean by check valves, unless you mean the residual pressure valves used on the non-servo dual master. Leaving these out can cause an initially long pedal on that master, but you shouldn't have to fit them to those systems that didn't originally have them (single-circuit and servoed dual master) to correct a problem.
PaulH Solihull

I've run into this a few times on cars that have had mods. You will need to do the math. Volume of MC compared to the volume+movement of the caliper/wheel cylinders. In most cases, you'll need a larger bore master to accommodate the changes in volume requirements of the calipers/wheel cylinders.

Kelly
Kelly Combes

Frank

I had this very same problem. I have Wilwood disc brakes front and rear and even after installing residual pressure check valves, the pedal was just ok at best. As mentioned by Kelly, the fix was to install a larger bore master cylinder.
Link http://forum.britishv8.org/read.php?7,31766 although my car is a C/B car.

Bill
bill jacobson

This thread was discussed between 21/02/2012 and 11/06/2012

MG MGB GT V8 Factory Originals Technical index

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