Jerez, Spain - Ferrari's Sebastian Vettel was fastest for the second consecutive day in Formula One testing on Monday but champion Mercedes again did the most laps despite a water leak on Lewis Hamilton's car.
Quadruple world champion Vettel, who moved from Red Bull at the end of 2014, did 89 laps before rain swept in at the Jerez circuit and was almost a second faster than the rest.
Reigning champion Hamilton did 91 laps on his first day in the new Mercedes, ending the day fourth fastest and adding to the whopping tally of 157 laps put in by his German team-mate Nico Rosberg on Sunday.
Hamilton said: "The priority right now is to put in the laps, not set fast times, and there were lots of positives today. We got good mileage and we hope to get even more in the next two days."
Brazilian Felipe Nasr was second fastest in a Ferrari-powered Sauber, with Finland's Valtteri Bottas third in a Williams.
Lotus made its first appearance of the season, after arriving late and failing to get on track on Sunday, with Venezuelan Pastor Maldonado completing 41 laps in the new Mercedes-powered car.
TOUGH DAY FOR McLAREN
McLaren had another tough day getting to grips with its new Honda-powered car. After Fernando Alonso completed only six laps on Sunday. Jenson Button managed just seven on his first day at the wheel.
Button's best - and completely unrepresentative - lap was more than 30 seconds slower than Vettel's fastest. Nevertheless, Button stayed positive.
"We expected this test to be tricky - and the next two days probably won't be plain sailing, either," he said. "But people have short memories. Look at last winter; the first test of 2014 was very tough for everyone too.
"So there are no real worries at the moment; the last run of the day seems to have sorted the problem out."
The team, starting a new era with Honda returning to the sport after a six-year absence, had worked overnight to rectify Sunday's problems but a new one emerged once the car was back out on the track.
Button drove a number of installation laps but the engineers then decided to investigate more deeply and believed they had found the cause, after which Button put in one more trouble-free lap before the day's session ended.
LEARNING CURVE
Honda's motorsport boss Yasuhisa Arai said: "Today was another tricky day, but we feel that we've overcome the main issue that affected our running during the first two days.
"Despite our lack of mileage, it's been an important learning process for our engineers, who've really started to develop a close working relationship with everybody at McLaren - that's a big positive.
Button has admitted he has no idea what to expect from the new McLaren-Honda MP4-30 this season after a difficult start to the campaign at the first pre-season test in Jerez.
"In terms of understanding our performance we're still at zero, we don't know where we are," he said. "You come into the first test thinking you will have issues and try to solve them. That's what testing is all about.
"We are not going to be setting the world alight with lap times or laps on the circuit. Hopefully we'll get to the first race and we'll be competitive, but we don't know where we stand right now."
McLaren's slow start on the track comes after a difficult winter for Button as he had to wait until December to find out if he would have a seat for a 15th consecutive season in F1, eventually edging out his team-mate from last season Kevin Magnussen for the right to partner Alonso.
Alonso's return to McLaren came as a surprise to many after the Spaniard's ill-fated season with the Woking-based team back in 2007.
The double world champion endured a frosty battle for supremacy within the team with then rookie team-mate Lewis Hamilton, whilst Alonso also played a role in the spying scandal that saw McLaren thrown out of the constructors' championship and fined $100 million.
However, Button believes the veteran pairing can work together in harmony in order to make McLaren a contender once more.
"Fernando is a double world champion, he's been around almost as long as I have in the sport, racing for many different teams and you pick up so many things along the way.
"That is such a great source of information when you have a new power unit in the car.
"It's a huge benefit for this team having us two here and I've always said I want the best guy to be alongside me. I feel we'll work very well together."
RED BULL LOSES WINGS
Red Bull also struggled last year when the new V6 turbo hybrid power unit was introduced but went on to finish second in the Australian season-opener, before disqualification, and ended the season with three wins.
The team suffered a setback in Jerez on Monday, however, when Russian rookie Daniil Kvyat damaged the car's front wing, leaving the team without a replacement. Kvyat continued without the wing for several laps.
Race engineering head Guillaume Rocquelin said: "As with everybody else the pressure to get the car ready on time for this test meant that we didn't have a replacement here.
"However, there are plenty of routine checks that need to be done, systems checks that go on all the time in the background, so we could at least get through a lot of that work today, which is why we sent Dany out for some laps without recording a time."
He said a replacement wing would arrive in time for Tuesday's test session.
The former champion, now without Sebastian Vettel after he moved to Ferrari, have been testing at Jerez with a car painted in black and white 'camouflage' livery.
Team principal Christian Horner said Vettel had worn a helmet with a similar dazzle design at the 2014 Italian Grand Prix and the team thought it would be interesting to extend the concept to the whole car.
Horner, who had said last week that the team were rushing to get the Renault-powered car ready for Jerez, promised the livery would be "even stronger" for the first race in Australia on 15 March.
Red Bull designer Adrian Newey said: "Everybody works away over the winter and once the new cars come out, then everybody is looking at what everyone else has been up to.
"If we can help to disguise that a little bit and conceal some of the shapes we've developed over the winter, then that prevents them getting as good a knowledge of our shapes as they might do otherwise.
"How much difference it makes, we'll see. But it's something different."
Newey repeated his view that champion Mercedes, which won all but three races in 2014, would be hard to beat this season and said the regulations favoured the engines over aerodynamics.
"I think Mercedes should win it, frankly. They have a power advantage and certainly in this relatively engine-dominated formula, then that puts them ahead," he said.
"Can we make enough of a difference on the chassis to mount a challenge? I think it will be extremely difficult but we will obviously do our best. A bit more freedom on the chassis regulations would make that more viable."
Reuters