[WA-Alfa]

RHD 105 GTV 2000 (1974).

My brakes work well and I have good brake pedal pressure while driving. However on one occasion the brake pedal went straight to the floor momentarily. I was decelerating while approaching a red traffic light and braked at the same time as depressing the clutch. By lifting my foot off the clutch and brake pedal and then braking again without depressing the clutch the brake pedal pressure returned and all was well. I had the problem repeat itself in exactly the same way again this evening. That's twice in 2 weeks - albeit with very infrequent driving.

I recently had my rear hand brake shoes replaced by a mechanic. Could something have gone wrong during this replacement to have caused my problem or is the problem more sinister.

 

[Andrew]

Seems unlikely related to replacement of rear e-brake shoes unless he messed up the calipers, pads, or lines.
After a little exploration, I'd probably go for a new master cylinder. As a RHD car, you might also look at your two boosters.
Andrew

 

[dfrye]

If the pedal does not come back to you, it is likely the piston rubber sticking in the master bore. Even i this is not the case, if the master looks scabby I'd be sending it off to be honed or sleeved and rekitted. For really good pedal everything needs to be good, but the mc is probably the place to start and its not going to cost much in time or money.

 

[WA-Alfa]

Well I know that it has been a while but today I had my mechanic replace the brake master cylinder and bleed the system. I have also replaced the brake check valve on number #4 cylinder as well.

Sadly this repair work does not appear to have fixed the problem. I took the car for a comprehensive test drive of about 500km. The problem is very intermitent and only occured 4 times over 500km and almost always occurs on deceleration when I am either depressing the clutch with my foot off the accelerator or when I am coasting down a steep hill. If I pump the pedal a few times then pressure returns to the pedal and all is well.

What further steps do I need to go through to diagnose the problem?

 

[AlfistiSA]

WA-Alfa

I'm with Andrew & dfry on this one, it sounds like the master cylinder.......

Brakes are the one area you DON'T want to skimp on; crunching the nose is guaranteed to not make your day.

Forget flushing it, pull the MC off and strip it, check it carefully then have it sleeved with stainless steel & fit a new kit - or buy a replacement MC from one of the well-known UK / USA / European vendors.

Years ago I had the opposite on my '69 RHD 1300 GT J where the brakes used to stick on & the only way to release them was to crack a bleed nipple on a wheel. pumping fluid through the MC produced a tin full of clean fluid at each wheel. When I pulled the MC it was a ball of rust & gunge inside with a passage made by the clean fluid which was passing through the muck. On principle I rebuild everything on any vehicle I buy and generally sleeve with stainless steel where required.

Decelleration would lower the vacuum effect to the booster, but not significantly so and in no way should it result in the pedal hitting the floor

We're also a RHD country & your car was probably assembled out here. I've had quite a few GTV's and GT's over the years, so am familiar with your standing pedal car. Does your MC have the reservoir with the bleed nipple on top ??

Ciao
Greig

 

[WA-Alfa]

Thanks Greig, but as I indicated in my last post I have already replaced the MC with a new one (this was done only yesterday) and yet the problem persists.